I followed these directions and still can't get it to work. When I try to migrate my old account, it says that I'm entering the wrong username or password. There is no function to retrieve a lost password, ironically.
I've sent emails to Mojang, but I doubt they are going to be able to help me. They say the best way to prove that you had the full version is to give them the transaction ID. Well, considering I bought this back in alpha/beta stages a few years ago and my credit card company only keeps 180 days worth of statements, it looks like I am screwed.
Cornbread Chemist