Puberty seems like a good analogy. It's jarring because it's instantaneous, but it's not like their brain gets wiped clean in the process. Other people have mentioned the show's charmander turning into an increasingly rebellious shithead, and the games describe some evolutions as having drastic behavior changes.
Their personality changes
Like when kids become angsty teenagers or totally a different personality?
Puberty seems like a good analogy. It's jarring because it's instantaneous, but it's not like their brain gets wiped clean in the process. Other people have mentioned the show's charmander turning into an increasingly rebellious shithead, and the games describe some evolutions as having drastic behavior changes.