Renaming user failed, lost admin rights
From I don&#39;t have administrator account on my mac


  
  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.)
  Mount the drive by typing /sbin/mount -uw / then ↩ enter.
  Remove the Apple Setup Done file by typing  rm /var/db/.AppleSetupDone then ↩ enter.
  Reboot by typing reboot then ↩ enter.
  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.