Oh that's what you mean. No it won't crash, but it also won't work like you expect. Animations bug out, and hitboxes may not be aligned either.
I've never looked into the game's files, but my amateur guess is that compared to Zestiria and Berseria, Xillia stores its animations in individual containers rather than in some shared pool. If your new Milla uses Overdrive, she could initiate it fine, but if the arte animation calls for 0x20, 0x21, and 0x22 (these are not real values, just examples), it will grab these animations from Elize's animation data because the base character is still actually Elize. Elize's animations stored at those addresses may be something else entirely, like running and sitting down, and therefore cause Overdrive to not function as intended. I'm only assuming this is the case because otherwise, we'd have more than just a basic Wingul PoC.