I had the same issue but a manual reinstall wasn't necessary. I believe the problem was that in the developer > profiles menu there is a certificate for beta updates that needs removed. Something along those lines.
That is correct. You have to unregister yourself and remove that beta profile.
I upgraded to beta 7 then it let me upgrade to 10.3. Since I was initially in the beta, I probably couldn't make the jump to the GM directly.
Sounds sane. Otherwise upgrade scripts would have to be exhaustive for beta1->GM, beta2->GM etc. So now there's 10.2->10.3 and beta7->10.3, much more manageable.