Like most people asking the questions here I am the newly designated BES "expert" in the middle of a major project. We are migrating two domains to one new domain. One domain is in the same forrest as the new domain and one is not. We are running BES 4.0.x on Exchange 2K3 on both original domains. The new domain will run BES4.1.x on Exchange 2K7. The problem is how to get everthing there as seemlessly as possible. I have two seperate SRP keys so I can do a phased migration.

We have done:

Moved BES and same forest domain users into new domain on exchange 2K3
BES 4.0.x

My idea for the moves and upgrades are as follows please let me know if they can be improved on or they are just bad.

Install new BES with 4.1 in new domain and move existing domain users to it. Leave them on Exchange 2k3 because that is what the timeline dictates. Upgrade original BES to 4.1 for redundency/load balancing.
Move outside forest users to new Exchange 2K7 server in new domain and move them to new BES server.
Move Exchange 2k3 users to Exchange 2K7.

So what did I miss?

Are both Exchange servers in the same Exchange Org? As long as the mailboxes are all in the same Org and you grant appropriate permissions on each Exchange server per the install guides you can move users between wherever / whenever.

The key with this will be to upgrade the current one to 4.1 and then connect the new server into that 4.1 database (running the same version of 4.1). then you can easily move users from one BES to the other.