After I migrated my existing account I had to change my login name to my email address rather than my profile name. I am fine with that, it adds to security.
However, after also upgrading to version 1.7.2 I have lost all achievements and have to start from scratch. While this is not that big an issue, it would be nice of you to retain the existing achievements made by the player and not just forget about them.
I also lost all achievements when I auto-updated to 1.7.2 from 1.6.4 on my Mac bookj running OS X 10.8.5. Note: I had previously encountered the bug where "The End?" was never granted, although I had passed through the portal (and later killed the Ender Dragon).