Underscore in domain name, sync issues & application access & migratie to Win2008 new domain

There is an underscore in our domain name (i.e. domain_dity.org) which I believe is causing sync issues in DNS and some application access.  Plus our IP scheme is 91.0.0.0 which is a pubic IP range that belongs to some other country and is causing weird stuff to show up in DNS.  There 2 local domain controllers, 6 remote domain controllers all Win2003 Enterprise, 1 Exchange server, SQL, Oracle and a buch of other sequal apps.  I've been given the ok to upgrade to Win2008 and Exchange 2007.  I'm thinking of migrating everything to a new forest and domain (i.e domaincity.org) and a new IP scheme of 172.16.1.0/16 to 172.22.1.0/16.  I'm having a hard time figuring out where to start.  I know that I have to create a trust between the old and the new forest but how do I set up the new IP scheme and DNS so that all the users can continue access the resources while I migrate 700+ users, workstations.  I was thinking of using ADMT and the Exchange Migration Wizard.  I'm probably taking on more that I can chew but can someone help me get started?