Exchange 5.5 update user account

Setup:
Domain A is running Win2K AD; Domain B is also running Win2K AD with Exchange 5.5 (users from Dom A and B use this as their mail server). Trusts setup and working between domains.

Problem:
I am collapsing Domain A into Domain B; AD moving users to Domain B. How do I update Exchange 5.5, once the users are moved, to point to the correct Domain?

Users in Exchange currently point to: \\domain a\%username%
After the move, Exchange mailbox properties should reflect: \\domain b\%username%.

I know this can be done by opening each mailbox and manually pointing to the new Domain, but I was hoping there was a way to script it out, etc.

Many thanks Gang!!

Not an Ex5.5 Guy,
Geekinz