It would be useful to add something on recovering from a failed migration (like my recent experience with neurodifferent.me).
If (as in my case) your followers haven't started to transfer, you can simply go to your profile via a web browser and cancel the forwarding. Your old account will then become fully functional again (though of course you can't attempt the migration again for 30 days).
Even if your followers have transferred to your new account, but you want to continue to use the old one as well for some reason, you can again cancel the forwarding. This will not affect the followers who have already transferred - they stay with your new account.