Chicago already had MJ's number retired at the time. He wore 45 for the final 22 games of the season and playoffs. Then switched back to 23 for the following season.
A little different than Kobe's situation.
It's not different than Magic's situation. Magic's number was retired in 1992. When Magic came back to the Lakers in '96, he wore #32 again. I don't think the Lakers officially "unretired" it (as they would have had to do if Shaq wanted to remain #32 when he came to the Lakers) because it wasn't a different player.