Exchange 2010 cross forest integration

This is really a follow up question to a previous asked one of my inquiries (https://www.experts-exchange.com/questions/26783820/Cross-domain-mailbox-migration-Exchange-2010.html). The scenario is we have an existing forest containing DomainA.com with Exchange 2007. We want to move to Exchange 2010 but at the same time we want to create a new domain and forest as we have some old lingering Active Directory issues and the fact that we don't own the "DomainA.com" domain name we cannot secure it with a certificate. I have created a new forest and in that DomainB.local. Exchange 2010 has been installed on a Server 2008R2 virtual machine which is a member server to DomainB.local.

The question is how to move mailboxes and eventually user accounts from DomainA.com to DomainB.local. I know for sure I could just recreate the user accounts and import the old mailboxes but this would incur a lot of labor and would require that ALL mailboxes be moved at once. There are 17 mailboxes and some are quite large (over 8GB) which means that moving them all manually would take a long time, which means email will be down for a long time which is not really an option. I have done some research and everything I can find gives instructions using ILM 2007 which I am not familiar with at all (http://technet.microsoft.com/en-us/library/aa998597.aspx). Does anyone know how I can at least tie the two Exchange servers together in order to move users one at a time? If I could give more than 500 points I would becuase I think this is a difficult and unusual scenario.