As others have said, if he didn't really follow the Braves, then its Ok. But if he went around saying that he was a Braves fan while they won 14 straight division titles, then switched to the Phillies, he's a bandwagoner. He can't even use the excuse that he can't watch Braves games anymore because they're always on TBS here in Philly and everywhere else.
Conversely, I live in Philly and it's hard not to root for this team and get caught up in it. Being from WNY, I never really followed a team, but following the Phillies is almost as fun as following the Bills. I can safely say that I'll probably be a Phillies fan for life, regardless of if I move from here.