Changing my administrator user's username has removed privileges
If it's not already, make sure your Mac mini has an up-to-date backup.

Then follow these steps:


Shut down your Mac mini
Power up your Mac mini and immediately press and hold the command + S keys
Keep the keys down until you see text on the screen
Now enter the following command:

/sbin/fsck -fy 

and press enter
Next enter the following command:

/sbin/mount -uw / 

and press enter
Now enter the following command:

rm /var/db/.AppleSetupDone 

and press enter
Finally, enter the reboot command

and press enter


Your Mac mini should reboot and you can log in normally, after which point the Setup Assistant will launch. Now you need to create a new admin user. Don't worry, your other user accounts should be intact.  NOTE: You should choose a new user name for this admin account.

Once you're back up and running you can go to Apple > System Preferences > Users &amp; Groups and select the Allow user to administer this computer checkbox for the 'grant' user.