Your stuck with that side till the tournment ends. Although that are some bugs where sometimes when the game loads up, eventhough you picked a light character, you end up with a dark character. It happened to me twice the other day.

I was wondering why Light side had more demigods but less favor, this post cleared things up for me.

Not to complain, but there really should be a warning window that pops up and explains that you cannot change sides in the pantheon once you choose.  I enjoy the DGs on the light side more than the dark side in generall, but when i signed up for my first pantheon game I felt like playing Erebus.  Now because of that one time choice I have to play this side until the tournament ends?

I am sure I will still enjoy it and it isn't game breaking at all, but a warning window would have been nice.  Maybe I will make a post in the idea forum.