No law, but it is an incredibly dickish thing to do (quelle surprise!). Also, maybe sort this kind of thing out before announcing a rebrand?
Twitter has been doing this for ages though. Plenty of other examples of handles being taken from people and give to famous people. For example, if you registered @snoopdog early or whatever, these handles were routinely re-appropriated.
If you sued maybe you could argue there was some kind of implied contract? Hard to imagine it's worthwhile though.