Aren't there generally stipulation in apparel agreements that prevent number changes from happening mid-season? Every now and again you hear about an NBA or NFL player wanting to switch numbers that become available, but to do so you need to compensate the kit makers for all pre-made, undelivered jerseys and front the money for refunds on current year jerseys already purchased by fans.
Maybe? I think it depends on the league. I've heard stories about baseball players paying other players for jersey numbers after being traded, so I do think it happens. Definitely think you're right in the NBA and NFL, but not sure whether other leagues allow it.