Pralix, what's the level of Muto Vim level of difficulty for changing the implementation of a guideline in a spell? In context, I'd like to have St. AVery MuVi his Dragon's Eternal Oblivion into a Demon's Eternal Oblivion - just to check that no one is possessed. This would be similar to changing a transformation spell that changed the target into a dog, rather than a cat.
Personally, I rate it as a "minor' change, in that the MuVi guidelines include "using a different guideline within the TeFo" as a Moderate change. (As such, using the same guideline, only differently, should be easier). However, the Minor changes say that you can't modify "the primary effect" of the spell. So, if "primary effect" means guideline, then it can change it. If "primary effect" means implementation, then it can't.
Unfortunately, I have never found an actual definition for "primary effect" in the book. The main problem is that the MuVi guidelines were imported from 4th Edition, and even then they were kinda vague.
Again - I like the idea of using MuVi to easily change how spells work, simply because I don't care much for buying the same spell over and over again, just to get it to work slightly differently. As such, I'm cool with having 1 main spell, and then several easier MuVi effects that you can use to play around with it a bit. (This is mainly for Vim realm changes, MuCo shapeshifting, and probably CrAu weather phenomena - in essence any guideline that you may want to implement a number of different ways.)
It also makes MuVi a viable thing to study - as otherwise you need to have a specialist MuVi guy to get into the levels at which it is useful. (Ie, if changing a guideline is a Moderate change, you may as well just learn the original spell over again, as it's going to be just as difficult to MuVi it. But if it's a minor change, the player can choose to take the additional difficulty roll and extra time to learn an easier spell. This creates more options for the player, which I consider a good thing.)
However, others have disagreed with my interpretation - mainly on the "primary effect" clause - which I can understand.