As a proficient PHP/MySQL programmer, I have to say that this is pretty outrageous.  Renderosity already has an "Edit Profile" page, and it would take all of fifteen minutes to enhance that page to allow username changes.  Checking for duplicate usernames is also trivial.  Now, if the username is used as a key in other database tables as jevans69 suggests, then the programmers should be reprimanded.  Also, if the username is stored in more than one place (gallery,store, freestuff, etc.), then this site is pretty poorly designed to begin with.

If you're so concerned about people abusing this service, then just allow a maximum number of username changes.

There's no excuse for charging $24.95 for a service that just about every other website in the entire internet does for free.