Changing the user login name should work. It's just a piece of text for people to use. UID is the thing that actually identifies a user to the system. Some config files can indeed be using the name and of course the user has a home dir etc.

You shouldn't edit passwd file by hand, but by tools like vipw or usermod. Remember that there's probably a shadow system as well, so you need to make sure they're both updated properly.

I wouldn't try to rename a user but create a new one and either disable or remove the default account. Removing it risks a new user with the same uid (file access etc) or login name (and e.g. sudo rights) getting added later.

Changing the user login name should work. It's just a piece of text for people to use. UID is the thing that actually identifies a user to the system. Some config files can indeed be using the name and of course the user has a home dir etc.

You shouldn't edit passwd file by hand, but by tools like vipw or usermod. Remember that there's probably a shadow system as well, so you need to make sure they're both updated properly.

I wouldn't try to rename a user but create a new one and either disable or remove the default account. Removing it risks a new user with the same uid (getting same file access etc) or login name (e.g. sudo rights) getting added later.