Random thoughts I've jotted down from doing several migration of mult-site 500's in an SCN to a SE and retaining the 500's for the PRI's
Start by making sure that none of the line groups are 0, and are unique from line groups on the SE or any other expansion
If you had to change line group ID's, be sure to change them in the ARS tables too
Make a copy of the 50:Main ARS, and change the 9N shortcode to go to the new one (the 50:Main gets changed automatically when brought into the solution)
If you had to change the line group ID's, change them in the ICR as well
I like to reset the security settings to match the SE
Change the Reject Action to Log to Audit Trail (for now, because I usually screw up logging in a few times, change it back later)
While you're in Security Settings, check the Unsecured Interfaces
If the 500 has any SCN lines, remove them
When you start the process to add the expansion unit, CHECK THE BOX TO RETAIN CONFIGURATION on the 500
You have to put in a password for Web Socket, use something consistent, so you know what it is down the road
After the 500 has been added, check the VM destination, the License Server, and uncheck Allow Direct Media path on the IP line between the SE and the 500 (both ends)