I would definitely add that a best practice that has been adopted is the removal of user #1/admin as a way of preventing site hacks. So any checks would need to factor that in and not default to UID #1.
One possible solution is:
1. Place a warning above the drop-down box.
2. Add an extra step to warn the user what they are doing.
3. If the user is already Admin then continue to keep their user as Admin to at least allow the option of resetting this or they would otherwise be locked out and would have to depend on reimporting the database or reimporting site configuration to fix the issue.
Also, adding a Drush command that would allow this value to be updated would help. A new command under the drush user commands would be ideal.
Definitely, thanks for your dedication to this project over the years. Best wishes to you in your new endeavors.