You'll be able to choose a new starter for the beta, and I think a new starter for the launch, too. Experience will be carried over from the beta to the launch, (not sure about from the Alpha to the beta since the accounts aren't the same.) The way the exp carry over will work is that the exp will be carried over but not your card choices (you get cards every time you level up). So if you're level 3, you've got two extra cards of your choice. These will not carry over, but the first time you go to the Arena after launch, it will ask you to choose both again.
So, as a nice bonus, if you don't like your choice during the beta, you can always choose something different when the game goes live. This is all in addition to the freebies we'll be giving Alphas and Betas. I don't know, Alphas, how about a pet or an avatar that looks like the "waiting sphere"?
"Scissors are overpowered. Rock is fine." -Paper