Update:
Please see another similar post http://myreinspace.com/public_forums/Technical_Support/64-25608-User_name.html
My original reply:
I think you may need to create a new account. - I suggest you touch base with Madison Norton of REIN first. - There may be some super-user update that he could do on your behalf but I suspect the underlying database structure may be limiting such a change.
From http://myreinspace.com/help/ :
How do I change my username?
Usernames cannot be changed once they've been selected.
... It will be unfortunate to loose the links to your posts if you have to make a new account.