I don't think there's strictly a rule against it, but I've always figured that teams "strongly encourage" their marquee players (read: players they mass produce jerseys and shirts for) not to change numbers on a whim, because then they have to manufacture new merchandise and market it. Slightly different situation, but I remember that during the WBC in 2006, Miguel Tejada wanted to wear No. 4 (his number with Oakland) on his Dominican team jersey, but was turned away from doing so because they had already made a ton of merchandise with No. 10 (his number with Baltimore) on it. Any piece of merchandise licensed by the MLBPA (which I'd wager is most of it, outside of the really really cheap stuff sold outside Wrigley) will say KOSUKE 1 on it to match his jersey. Furthermore, I think I saw where someone on this board tried to order a FUKUDOME 1 custom jersey/shirt and the order was cancelled shortly thereafter. The bottom line is that I think the Cubs and their official licensors will try and cut down on FUKUDOME 1 merchandise as much as possible, but the fans are a resourceful and free-spending lot and the Fukudome name is going to get its fair share of play on unlicensed/customized merchandise.