How to Break a Forest

I?ve got a branch office where the W2K domain controler is linked in a Forest such that it has a FQDN as sub.mydomain.com. In My headquarters, the FQDN is mydomain.com and the sites are linked trought a fiber link. Now the company wishes to release this link and here begins the problem.

Is there a way to take sub.mydomain.com from this forest and yet retain it as a Domain Controler so that we don’t have to re-create the domain with all of it?s settings as a standalone domain?