I recently migrated my Minecraft account with my Mojang account and now when I log in to minecraft.net and try to change my skin, it says I need to buy the game... But I already have the game. What do I do? And is there any way to unmigrate my account?
First make sure that you're logging in with your email address, not your old Minecraft username. Everything uses your email address now that you've migrated your account.
If that's not the problem, this is simply an account bug that we can't help with. For account migration issues that aren't resolved by that help article, you need to contact Mojang support as it says at the bottom.
(No, accounts can't be un-migrated.)