If you install a new macos version over a old one all useraccounts lost there admin rights.
I think there is a way in the singleusermode with dscl to get the admin rights back.
This should be a nice feature for applejack.
I would like to do something like this, but have to think through the implications of making it easy for someone to hack your computer. Granted, with single user mode, anyone with lots of knowledge can do that anyway, but I don't like the idea of providing a "script kiddie" interface for people to quickly be able to give themselves admin rights. It's probably a good thing to leave that possibility to people who have taken the time to learn how to do it. Not closing this, just putting it off while I think about the ramifications.