One of the Forum Administrators should be able to help you with that. I think you could use the "Contact Us" link at the bottom of any Forum page since you probably haven;t been set up for PMs.
Use this Contact Uslink to request the change. Ask them if you can have different login and display names (I think they are linked). Tell them your new name request, if it's available, it should not be a problem.
Which admin did you contact ? .... if he's not about it could take a while, but even the ones that are online are very busy so you might have to wait a bit, but they will get back to you