Can I safely change my account's username? My Mac username is set to 'John', and I hate looking at the capital 'J' and would like to set it to 'john'. I see the advanced option in the Users &amp; Groups settings, but I want to make sure that it won't break my setup? Do I need to rename my /Users/ folder? Would it break any programs depending on my home path?
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.