I don't think a MuVi spell can turn a spell into a completely different spell of an entirely separate Form.
That's what Mysteries are for. 
Look to your core rule book...
Concerning the hypothetical illusionary wall. A side note, or question rather. Who goes around "lightly touching" walls? I tap walls, push against them, lean against them, punch them, bump into them, and so forth. If i wanted to test it, it wouldn't be a ginger sampling of texture.
Just saying 
Yup! Then you can create a thoroughly amusing magus, who knows one Formulaic spell extremely well, with lots of Mastery, but he is really a MuVi expert with a minor focus in Hermetic Magic, and casts various MuVi spells to transform his One Big Spell into whatever he wants.
Well, that's embarrassing!
... And dumb! No wonder I forgot about it.
I was starting a new thread with almost exactly the same idea (I think this is derailing the OP... but hey, since I'm not the first one to sin ...). My thought was to create a magus with the aforementioned focus, plus affinity and puissant Muto and Vim, and Diedne magic. He then uses spontaneous Muto Vim to change his one (in fact, eventually four, second to fifth magnitude) mastered formulaic spells. Straight out of character creation (with just 104 xp spent in Arts, which Affinities turn into 156) he can then create any any non-ritual effect of level 10 or less without fatigue, and any non-ritual effect of level 15 or less by spending fatigue, in any Art combination. By the time he has accumulated a total 350 additional xp in those two Arts (about 100-115xp in each, plus the Affinity bonus) he can reliably cast any any non-ritual level 25 spell fatiguing himself, and any any non-ritual level 20 spell either without fatigue (with just a little luck or with exaggerated gestures and booming voice) or with fatigue but without voice or gestures. Not incredibly powerful, but incredibly versatile!
Some extras. Take, as additional formulaic magic, a R:Sight version of the Intangible Tunnel (great with a glove talisman!), and a D:Ring and D:Moon version of Maintain the Demanding spell. Take as your "basic" spell (the one that gets changed) some "General" spell, and take Adaptive Casting -- so whatever mastery you learn at one level applies to all levels! Learn from Mistakes is another great Mastery to take if you plan on eventually learning many.
Yup that's what i was thinking about, but it raises questions :
- no need for requisites, so you can turn any spell in another TeFo combination but keeping the same target. so it's very versatile, but what may be the limits, you keep the target, but can you change the other parameters (range and duration), if the base effect was 3, when the spell is changed, does the new base effect (based on another TeFo combination) has to be 3 also ?
- what about parma magica ? if a target is subject to a ReCo spell, that i change into a CrIg spell with my MuVi spell. what form is used ? Corpus ? Ignem ? Vim ? (I'd say Ignem as the spell is changed into a CrIg, but the spell was casted as Corpus and is modified by Vim....
- in fine, the guideline says the structure of the spell is affected not the things targeted by the spell. there are lots of interpretations possible of this point.... is the target not affected by the change of TeFo ? meaning it is affected by the initial spell as intended but the spell is considered being another TeFo (which wouldnot make sense to me i admit).
Am i the only one having a headache ? (i like theory crafting, headaches are a good thing 
)
No, Muto Vim is murky.
You're asking how to determine a penetration total. A MuVi specialist such as this is using magic for utility, and likely not offense.
You have some reasonable alternatives.
Casting a MuVi spell to change a spell could change the final penetration total on the resulting new spell to 0, as penetration is a calculation of total power put into the spell above and beyond the level of the spell. Muto Vim spells that change spells into different spells act on the level of the spell, and ignore excess penetration. This leads to a possibility of a Muto Vim spell of greater power to capture excess in the original spell and turn it into penetration in the final spell. I would, like the guideline, divide that excess amount by two, as well, and therefore you would rarely have a lot of penetration on the spell, and it's an arithmetic headache at the table.
Spells changed by MuVi could have a penetration based on the MuVi score, and not the original Arts used, or the transformed Arts. Is this reasonable? Sure, maybe. Keep in mind that this guy is casting two spells to get one off. In combat, this basically means he's a slow caster. He might be able to achieve some impressive penetration totals, if he has sufficient time. While the other guy might be able to generate impressive totals with less time. Yes, the MuVi spell might be mastered for fast casting, enabling the spell to go off in the same round, but that's going to put a dent in penetration by taking 10 from the total for the fast casting penalty.
I don't think it makes sense to base the penetration total off the new Arts, as those Arts weren't even involved in the casting of the spell and aren't really accessed in any meaningful way. The Muto Vim specialist is acting on his general knowledge of Hermetic Theory to change spells from one type into another type, not his specialized knowledge of a certain Art.
I would quickly rule that the MuVi spell would need to be fairly specific in its effect. So if you want to turn a Corpus spell to an Ignem spell, you need a "Corpus to Ignem" MuVi spell.
As precedent, Wizard's Boost and Wizard's Reach (ArM5 p.160) are Form-specific and seperate spells. So my thinking is that what the spell targets (initial Form) and what it does to the target (transform it to new Form) both must be specified in the spell when it is invented.
EDIT: The exact effect on a specific spell would also need to be consistent with the initial spell. So a CrCo spell would be changed to a CrIg spell (probably causing damage), while a ReCo spell that moves the target would probably be transformed into a ReIg spell that moves the warmth of the target (probably causing a slight loss of Fatigue).
Yeah, I was assuming this was the case, as was ezzelino, above, because he was talking about doing his meta-magic effects spontaneously...
That brings about fairly weak effects, as the spell being cast would be of low level (probably no more than 10-15 right after Gauntlet) and only being able to affect spells half that level (and that after reducing by a magnitude or two for range).
Call me unimpressed by what you can do with those spontaneous spells.
cough Diedne Magic can change that quite a bit. Still, I do see your point, and it intersects with the whole idea of penetration. Such a character probably doesn't have a lot of offensive capability, especially against beings with magic resistance. And a lot of magi can design effective spells to disable/defeat masses...
Of course if we assume doing it spontaneously it would not be very impressive (Diedne magic is a little overated i think, i have two magi at my table with it, they sure come in handy but still).
I agree, the MuVi spell would have a fixed effect (transform a spell into a specific TeFo doing a specific effect) else it would totally break the limits of hermetic magic.
But he point was to attack with illusions, the idea we had when we thought of it was :
I create an illusory fireball that lasts a Diameter then i change that spell to make it an attack spell (in game it could be made with fast cast, or with a magical object, or any combination, actually that's not my point as there are solutions).
For simplicity, let's assume we're CrIm some fire around someone, then we want to MuVi it into a CrIg effect (we could change it into PeIg or any TeFo as guidelines says but we'll keep it simple).
In theory we could have a level 15 CrIm (one sense 1, moving +2, animated +1, R : voice +2, D : Diameter +1, T : Ind 0), so we would need a MuVi 30 to change it into a CrIg 15. is it correct ?
I'd say the penetration total would be calculated on the MuVi Spell, not on the CrIm spell in this case. so in defense i'd look at the Vim of the target (and not Imaginem nor Ignem) does it look right to you ? (that's my interpretation as of now, and if i'm wrong that's not a problem for me, but i'd like it to be clear in my mind).
I know, that's utterly useless, having to master a level 30 formulaic spell and another level 15 to have a mere CrIg 15 as results, but i'm more interested by the theory.
Certainly, Diedne Magic would raise the level of spells by around 50%.
But as you say, his attack capabilities will be severely limited, as he would need to transform another spell to do so. Even defensively, he would need to fast cast anyway, further reducing his capabilities. Sure, other magi also need to fast cast their defenses, but they only need to divide their casting total once. Such a MuVi specialist would need to divide once to spont his MuVi spell, and that spell would only affect a spell of half that level.
Let's take an example of a magus with Diedne Magic and scores of 18 in both Muto and Vim (having both Puissant and Affinities in those) as well as a Stamina of 3. In a neutral aura, his casting total with Fatigue is 57 + stress die, so let's go for an average of 63. Minus 10 for fast casting. The MuVi spell he can pull of will be level 27 (rounded up). He probably wants to do this at Voice range, so his effective level is 22. Thus he can transform a spell of level 11 or less.
By comparison, most magi can pull off a fast cast defense of level 10 in their specialties, protecting themselves against level 20 attacks. Almost twice as effective as a MuVi defense (although admitedly a bit less versatile).
The concept is nice, I suppose, but comes up short when one tries to actually do something with it.
I would calculate the resistance total against the actual final form of the spell. Creo Imaginem spell which is then transformed by a muto vim spell into Creo Ignem is Creo Ignem. Resistance should be on the final form, always. Vim affects the spell (that's the target) and not the recipient of the original spell. What the penetration level is an open question.... Without a spell that accounts for the total "level" of the spell, including excess that goes into penetration, I feel that Muto Vim spells should result in a penetration of 0. On the other hand, I can also see an argument for the Muto Vim total determining the final penetration, and it's not really overpowered to any serious degree, because that's a MuVi 30 to transform that CrIm 15 to a CrIg 15, which would do +10 damage.
That's why he has Affinity with his one formulaic spell's mastery ability, so he can add his Mastery Score to the Penetration of every spell he casts!
FWIW,
This is the kind of thing that I consider Broken. Powerful? Maybe.
Does it break a standard assumption about how the game works? Yes. The biggest problem is that MuVi spells can change the TeFo of a spell; worse, it does so without requisites. Getting Ignem out of something that is not Ignem or that does not naturally produce Ignem warps the fundamentals of Hermetic Magic.
Anyway,
Ken
Has anyone done this type of MuVi change in game?
It's an option I've considered a few times but the spell levels and practical limitations just don't stack up as worth while. It's not imho powerful at all unless the character is highly specialised and then it's only useful circumstantially.
I tend to agree, it's not so useful in practice. To be at all useful as a utility spell you have to spont the MuVi, and then you can only affect a spell of less than half the level of your spont. I suppose a hyper-specialized MuVi magus, perhaps with something like Diedne Magic thrown into the mix, could make this kind of awesome but I've never seen it done.
I think only a very generous storyguide would allow a magus to use mastery abilities in a spell that had been "totally" changed. In my Mythic Europe the MuVi caster had better be prepared to avoid botches on both the original spell and the spontaneous MuVi he casts on it!