Fantasia Canonicity

[Iscah]

I can't remember where but, lore-wise, I read that Fantasias not only change you but change you retroactively, i.e. your character's history changes to that they were what they changed into.

That's not what it does lore-wise, it's what it does functionally. If you rewatch early cutscenes after fantasiaing (or any sort of visual change to your character) you'll be as you are now, not as you were at the time, because cutscenes just have a big space marked "insert player character here".

If anything, that's proof that it isn't something that actually happens to your character but an out-of-canon game function.