How to move an Exchange domain to another AD forest

Hello Experts,

I have a new project for the early part of next year which involves the "splitting" of a company, as such they want to remove one of their Exchange accepted domains and add it to another organisation in another AD forest. There are too many users to simply "flick a switch" and migrate across in one go so there will need to be a migration period where some users on the new domain have mailboxes on the old system using Outlook Anywhere and some will be moved to the new mail system. The organisations are physically located at different offices but linked by fast, fibre VPN links. From a high level view I see my options as either;

1. Setup the new AD domain with the new Exch, add users to the AD and Exch, import their data to the new mailbox from their local OST file, delete their mailbox off the old Server and create an SMTP connector to pass unknown local mail to the new Exchange Server, then once all users migrated simply change the MX record to the local circuit/IP address.

2. Setup a trust between the organisations, try to carry out mailbox moves, once all mailboxes, users etc moved to the new domain simply break the trust and remove the accepted domain from the old Server.

Any suggestions/guidance? Not too concerned with specific detailed steps; (unless someone has them handy?) but more pointing in the right direction. I don’t think public folders etc a concern in this instance, only email/mailbox data needs migrating.