Selectable use of *Extend Thing options

[Redcap's Corner]

I have a custom subschool of magic, and I want it to be optionally retroactively applied to a number of published spells. I made the subschool without issue. I was able to use the "*Extend Thing" tab to implement the addition without issue. The problem is that I'd like to set up the retroactive application of this subschool so it's only used on characters that select the option from the source list, same as many published optional subsystems are selected. *Extend Thing doesn't seem to support sources, and it's my understanding that I couldn't cheat similar *Extend-esque code into the program without burying it in an option that would have to be added to the character somehow. Is that right?

Also, on a related note, is there a negative equivalent to the *Extend Thing tab? Some of the spells that would retroactively gain this subschool would need to lose an existing subschool, but I can only add tags with *Extend, not remove them, right?

Is the answer just to make a file with a bunch of modified versions of existing spells that overwrite the originals, setting each up with the custom source that would allow the subschool to be retroactively applied?