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.