My username is Roganjosh348 and I wish to now use PvtJohnTowle
but I cannot change my username

http://www.gametracker.com/account/manage/edit.php

When I try I get the following error message

Username contains invalid character(s). Only the follwing characters are allowed: a-z 0-9 _ -

I am not using any characters or spaces. What is the problem and how do I change my username?

Thats cause you use upper case letter, put it just like this pvtjohntowle