Help with domain mapping- non-multi-site

I asked this earlier, but I guess I only get 1 answer even though it didn't work. So, here goes again:
I created a sub-domain and got it just the way I want it. Now, I have a pre-existing url that I switched the nameservers to a different add-on domain as directed by the hosting service.
My problem is, everything is still on the sub-domain section, so I redirected the add-on to point to the sub-domain.
I need to somehow get the sub-domain files to show as the main site and url.
This is NOT a multi-site, so the domain mapping plug-in won't work.