New company aquisition

My company ABC just recently purchased another company XYZ.  The XYZ company is shutting down completely 100%, they still will be needing to receive emails to the XYZ email account for about three months.  Now since their main facility is no longer going to be there, and they will be closing their shop up completely.
I know I need to move their MX record from their old IP address to one of my open addresses.  

Here is a Break down of the two systems:
XYZ.com is on an internal exchange server 2007
only 4-8 users will be staying
They have hundreds of customers that will still need to contact them due to they were a vendor.
They are located in Austin, the users will transfer to our Austin Location.

ABC.com has an internal exchange 2010 server which provides email to Users in Houston, Austin and various other locations.

We have lost our tier 3 exchange individual, so it was pushed upon us Tier 2 individuals to handle this.

What would be the easiest way to do this?  I have done this before but only part of the transition, and it has been almost two years so I have forgotten most of the items.
The major hang up, is that the ownership doesn't quite understand IT, and thinks this can be done in no time.  So the time limit has been accelerated drastically.