The "Anything "worse" must be Perdo" paradigm is so flawed I don't even know where to begin. It's not that simple, nor that restrictive, and I truly wish people would stop applying such an over-simplified and blindly narrow interpretation of one aspect of that Technique's definition to anything and everything that could possibly be interpreted as changing something "for the worse".
Change a boxer's hands into soft fluffy pillows - very bad, much worse, very fail - Muto.
Change a sword into a feather - very bad, much worse, def fail - Muto.
Change a person's body to liquid or stone - fail - Muto.
Change a spell so it isn't effectively what the caster intended - Muto.
This isn't "killing" the spell any more than turning a body to stone or water is killing the person, or turning a sword into a feather is "destroying" the sword. The person is still quite alive, the spell is still quite active, just both so different as to be unrecognizable. Almost certainly "worse", and inarguably Muto.
No, but changing a living person into a dead person IS perdo.
Thus, without specifics, "living spell to dead spell" is perdo vim. As I said, "changing a spell so it's so inconvienent to the caster is muto" - but it's a side effect of the change. Just cancelling it out, without any specifics other than "it's now broken" is Perdo.
Hands to fluffy pillows? Of course it's Muto. The defense to being punched is a side effect. But that's a specific change, and the boxer could still kick you (or smother you, for that matter). So it's not a complete defense. Similarly, you could muto his legs to jelly - thereby making him immobile. OK, fine - but the equivalent effect in Muto Vim is an Inverse Wizard's Reach (-1 to range). If you're within shouting distance, you will still get hit. So it, too, is not a complete defense.
Change the boxer into a statue? Sure, perfect Muto defense against being attacked by that boxer. But that's a side effect. Therefore - what's the change that fits within the muto Vim guidelines that allows for that perfect spell defense? IMO, Wizard's Fork + a decent Parma comes closest. But just saying "turn spell that works into spell that doesn't" isn't Muto. You have to be specific, and gain the benefit of the defense as a side effect.
This is a fair point, which is not really adressed by the spell at this point.
What might be missing from it (to satisfy Kevin's argument) might be how the target spell is destabilized, and the effects on it if the caster succeeds in controlling the spell.
What if, for example, the MuVi spell were to destabilize the target spell by injecting more power into it, the result of retaining control would be that the spell ends up as more powerful or distorted in some way. The actual effect might be up to the storyguide (or random) and would be consistent with things that MuVi can do (increased R/D/T, more damage, added penetration, change of Form).
One I can think of would be (to use the "change boxer into statue" analogy) would be a sort of "freeze spell" effect. That is, the spell is literally frozen in the air, but thaws out later. Effectively, it's forcing the spell to accept a ReVi "Delay effect" watching ward. And, if the delay (say, Sun) is sufficiently long, the target (ie, the mage) could vacate the battlefield, and have all the spells fizzle out due to lack of an in-range target.
I would say this is probably be considered a "significant" change. (ie, it's not changing the TeFo, but it is making the spell significantly more complex, as it now has its own built-in watching ward.)
Alternately, the spell itself is simply "redirect into a pre-existing watching ward" - in which case it would probably be superficial, but you need to have a watching ward waiting to "catch" the spell.
EDIT - or, alternately, it might simply be what happens when you fast-cast a Watching Ward with an uncooperative magi.
EDIT II - yeah, now that I think about it more, this is more likely a straight-up ReVi effect - It would likely use the same guideline as a normal Watching Ward, but would have to beat the Magic Resistance of the uncooperative spellcaster, would have to be at voice/sight range, and would have to be fast-cast.
Ah, well - it's still a good idea, IMO - if you've got a ReVi specialist.
The specific effect I have in mind now for the spell is to alter it so it is cast with zero Penetration. Same target, same all of it, but zero pen. That'll make any spell cast with this effect bounce of parma. I was hunting for a way to make a spell not effect my magus and that'll work.
edit: Just needs a funky name now. Blunt the Conspicuous Spell, hmm. nah.
After reading the comments here I think that is specific enough to be applied, and also not a monumental change in the overall spell.
Hm. Well, halving penetration is basically Wizard's Fork, and that's keeping the total amount of skill rolled on the table (sort of - split between two effects). Although the benefit seems to be that it doesn't need to be based on a Form.
Personally, if I were to be ADDING penetration to an effect, I'd use the general guideline of "significant change" equals +5 penetration, while "total change" equals +10. Although thematically, magnitudes do seem to represent something different than just a flat-out skill roll.
Regardless, I would argue that REMOVING penetration would be based on a similar level of difficulty. Although I'd argue that, in keeping with the Wizard's Fork effect, you wouldn't' have to have a Form-based technique.