It is recommended you do not change the domain name during the migration process but should you want to there is some documentation in the following link:
http://blogs.technet.com/sbs/archive/2009/05/01/sbs-2003-to-sbs-2008-migration-to-a-different-domain-name.aspxIf doing so the better method would be to purchase a migration kit for your scenario from
www.swingmigration.com for $200 which includes unlimited personal support for 90 days.