From what I've read, you'll have to wait until the current tournament is over.
I HOPE that we're able to change after that, would love confirmation of this though.
This is probably the most annoying thing for me (besides the obvious connection problems), because there is never ANY NOTICE that the side you are picking cannot be changed later. I would have spent much more time in single player seeing which side I preferred before chosing a Pantheon team had I known that.
I really hope this is taken into consideration, and that players are forewarned in a future patch.
