Essentially, you are looking to perform a logical consolidation by collapsing the 456.com forest/domain and folding it into the 123.com forest/domain. Exporting and re-importing the data comes at a high cost and in terms of the time that it takes and its impact on the users as a result of the outage. I understand that you are working with limited resources, so the problem will have to be approached with some degree of creativity and flexibility.
Considering that you are collapsing environments anyway, and in effect, migrating users from 456.com to 123.com, why not migrate users, groups and computers using the ADMT. You can do so into specific OUs temporarily which would allow you to manage them separately, which will come in handy as I’ll point out later.
This would allow you to use a swing method approach for the migration where once you free up an Exchange mailbox server in the 456.com forest/domain, it can be rebuilt into the 123.com forest/domain and the users that were migrated in the previous steps simply moved to their destination server. You could then repeat this process until the required number of servers have been rebuilt.
The migration would be a little easier to manage with less room for error (exporting and importing data and importing user attributes) and the impact to the user community greately reduced; however, adequate network infrastructure and bandwidth to support the migrations must be available.
I am not sure how you intend to preserve the mail stores of the environment being collapsed since you intend to reuse the hardware. You can certainly make online or offline backup copies of the databases. My suggestion would be to make offline backups prior to commencing any migration work. This way the databases are already committed. Information can also be plucked from the information stores directly using OnTrack PowerControls for example.