I mean... I'm not sure about how it works either, since I technically don't "own" this AU (because it is very similar to Pokemon ReBurst and it's kind of an unoriginal conceptBut if it was up to me to make up the rules, I'd say it is possible for an Eevee (or any other pokemon) to evolve while fused with a trainer.Like, if a trainer battles while fused with their Pokeon, the Pokemon still gains experience, so they can still level up and evolve.If an Eevee evolves into a Jolteon WHILE fused with their trainer, their fusion design will change to look like Jolteon. When they 'un-fuse', the trainer will be the same, while the Eevee will remain a Jolteon! XP Not like it can 'de-evolve' or something (unless it's Mega-evolution)