As people may or may not know the Muto Vim spell guidelines for the first printing of the ArM5 rules were errataed by adding the paragraph "A Muto Vim spell, like any other Muto spell, can only change its target for as long as the Muto Vim spell is in effect. Thus, its duration should normally be at least as long as the spell that is its target. The spells below have a duration of Momentary, but versions with longer Durations can be invented as normal." at the end. This is the text included in the second printing of the rules.
This was then changed again by errata, by removing that paragraph again and replacing it with
"Muto Vim spells work by altering the magical energies that create the spell as it is being cast. The spell is the result of the combination of the base casting and the Muto Vim effect, and has its effect once both the casting and the Muto Vim effect have finished. This means that a Muto Vim spell must have a duration at least as long as the casting of the target spell, but need not last for as long as the spell itself. For normally-cast formulaic spells, a Momentary Duration is sufficient, but if the casting time is longer for any reason, the Muto Vim spell must also have a longer Duration; Sun is sufficient for any practical Ritual."
Adding this also meant the adding Wizard's Vigil/Day of Communion spell as replacements for Wizard's Communion when used for rituals.
Now to my question? Why were any of these changes needed in the first place?
If you compare the current wording of the MuVi guidelines to those in the first printing of the rules the practical consequences of the errata are that you need to use D:Sun instead of D:Momentary to affect ritual spells with MuVi spells. And that's it as far as I can tell.
A lot of extra text, need to adjust many things regarding Wizard's Communion, just to make sure one needs slightly higher level MuVi spells for use on rituals.
It all seems pretty pointless to me, but perhaps there is something I have missed?
The original version fundamentally broke the way that Muto worked in the game, by allowing Muto Vim to have effects that lasted longer than the spell duration. That's an important part of the distinction between Rego and Muto, which needed to be clear because in earlier editions it was not. There were two attempts at getting it to work with minimal changes, which is why there were two sets of errata.
Did they? I was under the impression that the original text said them to have a "nominal duration of momentary" with an actual duration as long as the spell they affected? That's a difference in how the magnitude is calculated, but it doesn't break how muto works as far as I can see
Both the original and the current text of the MuVi guidelines say:
"Most Muto Vim spells last as long
as the spell that they have altered, with a
nominal duration of Momentary. It is not
normally possible to have a Muto Vim
spell that lasts longer than the target spell."
Which I agree would seem to neatly solve any problems with duration, and leaves me even more mystified why either errata was deemed necessary.
Erik Tyrell started off with a good point about Range. You cast a spell with Momentary MuVi bumping it from Voice to Sight, for example. This allows it to put an effect on a target far away. Why, now that the target is affected, does the spell suddenly bounce back, still active but with a shorter Range? And how does that even work? If the target gets closer the effect reactivates? Meanwhile, the core rules point out that once an effect is active, it will run on its own regardless of the Range the target moves to.
ExarKun makes a great logical point as well. MuVi cannot affect already-cast spells. So after the casting, the MuVi has no effect. If that's the case, the MuVi spell ending early shouldn't matter. So the original erratum created a paradox.
Besides, people arguing about how it messes with Wizard's Communion are in even worse shape with the old erratum. For example, with that you needed a Year-duration version of Wizard's Communion to work with AotH, which would make that version of Wizard's Communion itself a Ritual.
Oh, I quite agree that the original errata can be problematic, and that the second errata was an improvement over the first errata.
What I don't really get is why there was any need for any errata at all in the first place, and why the second errata was considered an improvement over the original (pre- any errata) version of the text.
The part of the second errata that talks about how a MuVi spells affect the casting of another spell, and ceases working once the other spell is cast - that seems reasonable and fine.
It is the second part of that errata which I find problematic, where it requires longer duration of the MuVi spell when used on spells with long casting time, since the text in the beginning of the guidelines talk about how MuVi spells have a nominal duration of Momentary - suggesting that the actual duration may well be different. So the need for an explicit D:Sun on some MuVi spells seems completely unnecessary.
Unnecessary and adds complication, since without that you could just have all the MuVi spells with a nominal duration of Momentary. Which would have avoided the need to add Wizard's Vigil to the game.
When the authors and editors developed the spell guidelines from 4ed, they seem to have simultaneously developed an allergy to «special» durations (and targets/ranges). There used to be a lot of those, and they all made sense. Your nominal duration sounds just like one of those special durations they have worked so hard to get rid of ...