Different Person, Changed Person, Changed Man
noun
A separate person is someone who is different from who they were before. I saw you a year ago and you were someone else. Now you're a different person, in a good way! You're a changed man.
べつじん
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
コウイチはお酒をやめてから、すっかり別人になったね。
Koichi has become a changed man ever since he quit drinking.
友人のコウイチだと思って声をかけたら、別人だった。
I thought it was my friend Koichi when I called out to him, but it was a different person.
コウイチは昔はAKB48をよくディスっていたのに、今はすっかり大ファンになっちまった。まるで別人のようだよ。
Koichi used to diss AKB48 pretty often, but now he's a crazed AKB48 fan. It’s as though he's a different person.