Change the administrator log in long name?

It is safe to change the user's long name. You just have to be careful if you want to change the short user name. Here is an Apple article on how to change the user name:

https://support.apple.com/en-us/HT201548

I don't recommend changing ANY Users names. Too many users end up locking themselves out.

Instead, create a NEW username that suits you, name it an Admin User, and log in with the new name. Copy your files form the old User. do your regular stuff, knowing that the old user files are there if you need them. If all is well in a week or so, then delete the old user and its files.