we have a VM technician scheduled, my question is about getting the timings right for our home security, i.e. should this be after the VM change
A common VM switchover process is to send a VM technician to do the switchover and move the phone connection to the back of the VM hub first of all. Often as a separate follow-up visit VM sends a technician to modify your home phone wiring so that your existing home phone sockets (presumably including your alarm system phone extension connection) become linked to the VM hub's phone socket (but often you need to make this clear to VM you want that doing as part of the free switchover process).
What is the ADT technician planning to do that would conflict with the VM changeover? Some sort of conversion so the alarm uses a mobile signal to dial out? Or something different/some other reason you are anticipating a problem?