Yuyuza, I could be wrong, but I think on mjs first retirement the bulls retired his number immediately. So when he came back it had to be unretired. Which it was, in 95-96, iirc
He took 45 because the Bulls had already retired his #23 and he didn't want to un-retire it, but midway through the playoffs the Bulls were getting their butts kicked by the Magic and he literally borrowed a jersey from a fan (at least that's the story I heard) and played in his old 23. The league fined him for it because a player can't just change numbers midway through the season.