If admin account name is changed and converted into standard account , enter in single user mode .
See this article Start up your Mac in single-user mode or verbose mode - Apple Support
Execute this command .
mv /var/db/.AppleSetupDone /var/db/.OldAppleSetupDone
Then press enter key after that .
exit
Restart the Mac , then it will prompt language then create a new user account .
Note : In the first terminal command there is space after mv and also space after AppleSetupDone .