Hi robcraw,
That move to another subscription certainly includes updating that serial number. This isn't something that can usually be changed from inside the file.
Because the other business is on a lapsed subscription, this would need to go through our Data File Repair team as a serial number change request first. Once that's done, the team can help move the file onto the active subscription and get it reactivated.
The best next step is to lodge a support request through My Account. If you're doing this for the client, the request needs to come from the Primary Contact or Billing Contact on the account.
Regards,
Genreve