What's up with this:?
# Migrate users, computers, and security groups from your SLD to a new root domain with a fully-qualified domain name in the same Active Directory forest
# Migrate users, computers, and security groups from your SLD to a new root domain with a fully-qualified domain name in a different Active Directory forest
Exchange 2007 doesn't "have to be" in a root domain. You can have an empty root with a child domain or another tree and install Exchange there as long as PrepareForest and PrepareAD have been run in the forest root. You can then do PrepareDomain on the child/tree domain and install Exchange there. Why are you saying you have to migrate everything to a root domain?