I'm very much interested in metamagic, and following the link tot he other thread I saw I had actually also answered that back then.
There are differences regarding 'uravelilng (Perdo VIm) and *deflecting/suppressing' (Rego Vim) in the guidelines compared to changing.
The unravelling and suppressing have both a generic guideline, affecting half of mag+4 of target spells as well as a specific version affecting mag+4.
Generic version is like Winds of Mundane Silence. Specific version is like Unravelling the Fabric of [Form] and needs to specifiy both tradition and Form/type
But magic to change magic (Muto) works differently. Guidelines speak of *superficial * , significant, and total changes.I'll focus on the significant changes, thie guideline affects spells up to mag+1. Using the most common parameters of Voice/Mom/Ind means the +2 mags for Voice causes spells like this to affect mag-1 spells.
This is in line with "Wizard's Boost" and the variants hereof. However these example spells are Form specific - not something the guideline mentions!
And quite importantly - and mentioned earlier in the thread - Sorcerer's Fork does not need to the Form specific!
So, if I want a spell to make the significant change of changing the target (small "t") of a spell cast to affect someone/-thing else (may even the caster?), and I only look at the guideline and not at any example spells I get this:
Curse the Face in the Mirror
MuVi Gen
Voice/Mom/Ind
Causes any* spell being cast, up to one magnitude lower than the MuVi spell, to affect the caster rather then the intended target**. Spell needs to be fast-cast and this spell needs to have penetration in excess of the target spell***.
(Design: Base guideline for MuVi 'significant change', +2 Voice)
Notes:
*)There seems no rule in the guideline, and by extension from the example of Sorcerer's Fork, that said spell need to be specific for tradition (e.g Hermetic) or form/type!
**)Actually, there is no reason only the caster can be targeted, any legal target should do. There is no basis to demand the spell be that specific.
***) I really don't see why this is needed...or maybe I do? Because if this spell is generic rather than specific (like Perdo or Rego spells often are, in order to use a guideline equivalent in power to this) this sort of balances things out.
Exampli Gratia: Defences against a Ball of Abysmal Fire lvl 35:
A spell to specifically Unravel Hermetic Ignem needs to be level 25 to be sure to work (affecting level+2 mags+stress die), or 20 on average roll.
A spell to Unravel this , and anything else level 35 generically needs to be level 60 (affecting half of [level+2 mags+stress di]) to be sure to work, or 55 on average roll
A spell to deflect specifically Hermetic Ignem spells needs to be level 25 to be sure to work (affecting level+2 mags), no dice are involved. Example spells in HoH:Soc p129 used, except I use +2 mags rather than +10. Also, if the guideline says level+2 mags (or +10) then were to I "apy" for the +2 mags for Voice? Is there a mistake here?
A spell to deflect this, and anything else level 35 generically needs to be level 55 (affecting half of [level+3 mags]), no dice are involved. This guideline is actual in ArM5 ReVi guidelines, stating half of level+5 mags, compensated for +2 mags for Voice.
But a Muto VIm spell to affect any spell, sending it at another target needs to be level 30 (affecting level+1 mag, but compensated with the +2 for Voice). However Penetration needs to exceed that of the target spell. So while you need a spell of roughly half the level when compared to Perdo or Rego, you need a much higher Casting total. So Re/Pe needs to cast level 55-60+ spells, with Penetration irelevant and Muto needs to cast level 30 with a suitable Penetration so maybe they need the same range of Casting totals?
Also, some references say "with a level less than" but as Errata changes this to "equal to or less than" that is what I calculate with.