How can I reverse migration since my old MacBook took the place of my new MacBook?

MacOS is inherently a multiple-User system. if you choose to create only one user account, it continues to operate the same way.

When you invoke Migration assistant, there are MULTPLE options for bringing over your old files, and you are unlikely to have "accidentally" clobbered the newer files on your Mac unless you were really not paying attention when you did this. It puts up multiple warnings makes you take affirmative actions to overwrite your existing files.

What is more likely is that you have accidentally created an ADDITIONAL user Account, and imported the old files into that account.

User accounts on your Mac are enumerated and controlled in the User&Groups control panel.

User account Home folders are stored in the /Users folder.

First, check to see if your old files have been placed in a different user Account.

First, you need to determine whether your new files were clobbered, or whether the Old files were inserted BESIDE your existing files. That is why I took the time to explain how to look for another Account.

If you did manage to clobber your newer files by importing your older files, there is no undo. You would have to restore files from your Trusted backup.

¿What is the date of your most recent backup, and by what method?

I'm a bit confused. I'm reading your post and getting the impression that you accidentally migrated from the new laptop to the old laptop. However, like @Grant mentions, macOS usually puts up warnings....plus I'm not even sure Migration Assistant would even allow it unless the old laptop was running the same OS as the new one since items like Music, Video, Photo, Mail libraries would be incompatible going to an older OS.

Another really important question.....do you have a backup of either computer? If not, then the first thing to do is make a backup of both computers.