I was wanting to change my username and did, then I realized I could just change the name of who I post as and tried to immediately change my username back to what it was and it says, “Sorry, that username already exists!” which was me before. Is this cached somewhere on the wordpress side or am I doing something wrong?
Once you change your username you will not be able to change it back and the old name will not be available for you or anyone to use. If you just want to show your name differently please only change your Display Name. http://en.support.wordpress.com/change-your-username/