Apple - Renaming user failed, lost admin rights

From I don't have administrator account on my mac

1. Boot into Single User Mode: Start/restart your Mac. As soon as you hear the startup tone, press and hold ⌘ + S until you see a black screen with white lettering. (If you end up back on the login screen after a flash of the black screen with white lettering, enter your password and it will return to the black screen.)

2. Mount the drive by typing /sbin/mount -uw / then ↩ enter.

3. Remove the Apple Setup Done file by typing rm /var/db/.AppleSetupDone then ↩ enter.

4. Reboot by typing reboot then ↩ enter.

5. Complete the setup process, creating a new admin account.

This should allow you to create a new admin user.

I will edit this answer to include a fix for your old user soon.