Can I safely change my account's username?

Only if you are on a case sensitive filesystem (which OS X supports, but doesn't enable by default) and the program was coded poorly, would it capture the exact path to your home directory. The vast majority of programs use the proper manner to find files relative to your starting home folder and are immune to even more drastic name changes.

I say, back up your files and have a stab at changing the short name to your liking.

It should be safe to change your username from John to john from within Users & Groups setting without having to make any further changes via command line.

OSX uses a full name and an account name. You can easily change your full name under:

settings > users

but I am not sure if this solves your problem. Changing the account name (that is associated with your home folder) is more tricky. You can do it though. Apple has a page describing how to: http://support.apple.com/kb/HT1428?viewlocale=en_US&locale=en_US