Mail migration help

Hi,


I have been given the task of migrating 120 users to a new Exchange server and domain.

The change is needed as our sister company and us are merging  into one.

There are two domains in question lets call them *sister.com and *brother.com.

*brother.com users will be changing there addresses to sister.com addresses. However still need to be able to send as *brother.com once migrated.

The 120 *brother.com users are split up in 7 different countries in the world. I need to d o a phased transition for all users to the sister.com domain with little interruption. .

Chances are I will do 2 countries a week.

My initial plans are this -

Create all brother.com users on the sister.com mail server with sister.com (as default) and brother.com as an alias.
Point the MX record of brother.com to the sister.com mail server
Create a domain called brother1.com and assign it as an alias for all users on the brother1.com exisiting mail server.
Forward all e-mails for brother.com users to brother1.com
Migrate users (phased approach)
remove forwarding rule once user has been migrated..

Can anyone see any problems with my plan?

Thanks