Several people have Pokémon at level 100; they should be able to tell you about how that works.
To be honest, it looks a little like puberty. [... Nice analogy there, Hope.] I have three Pokémon from the Eevee line, all in different forms, and they tend to react the same way at first; disorientation and confusion. Sometimes they tend to forget that they have new abilities available to them, but they adjust quickly. As for the trainers, it all depends on how well you're able to adapt to sudden changes. For the most part, the Eevees' base personalities don't change, but are simply added with new quirks by their forms.
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To be honest, it looks a little like puberty. [... Nice analogy there, Hope.] I have three Pokémon from the Eevee line, all in different forms, and they tend to react the same way at first; disorientation and confusion. Sometimes they tend to forget that they have new abilities available to them, but they adjust quickly. As for the trainers, it all depends on how well you're able to adapt to sudden changes. For the most part, the Eevees' base personalities don't change, but are simply added with new quirks by their forms.