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.)
I had the same issue when Minecraft migrated to Mojang global service a few years ago.
The fact is you have two possible logins for Minecraft:
The first one is on the Minecraft website.
The second one is on the Mojang website.
Try connecting to the Minecraft website first, make sur all information are valid (try to connect using user name, then try to connect using email). (I don't remember if there was an option to migrate from the Minecraft website so if someone remember feel to to edit.)
If both worked fine, then try to connect on Mojang website using the email or the username. The login is case sensitive so be sure you did not forget that on the username.
If you still can't connect to the Mojang website:
Try creating another account on the Mojang website (you do not need to buy Minecraft again), then try to add you Minecraft information with that account (in the profile settings).
If both of this did not work, then you should contact the customer service.
The work around I gave you was the manipulation the customer service made me do when I contacted them 3 years ago because I had an issue with the migration too.
Best Answer
There are more possible answers:
Reconnect it. Make a deal with your guy.
Buy a new account. That's what accounts are made for.
Don't do anything (Missing ugly smiley here). Play the demo all the time or don't play MC.
So, you need to be logged into a account - Your friends, or your own new one.