from what I have found you have to create a new forest and use the ADMT (active directory migration tool) to be able to migrate the domain to the new one. I found this article on it:
ADMT is the primary tool for such migrations.
Key points about ADMT:
Admin privileges on both domains, install agent on Source (old) domain,
two-way (external) trusts, external trusts require NetBIOS resolution
which may require WINS server (and all DCs set to use it) if DCs are
not on same subnet, put your Destination Domain Admin account into
the Administrators group of the Source domain.
this information was posted in 2005 by Herb Martin at:
since it is old I will need to do some more research on the subject to find some more recent news on it.