You moved into a place with a pre-installed home security system
Sometimes when people close on a house, a modern security system is already established. All the devices work, are hung in the best places, and are ready to go. In this example, you might see that you have to call and start a new contract. A Vivint professional will still come to your home to ensure there are no integration problems, and they could offer suggestions on upgrades. It's a fast trip, and you can have your monitoring set up without worrying about any new alarms and cameras -- unless you get some new devices.
The previous owners can give an existing Vivint contract to you for a one-time transfer fee to make your life easier. If the previous owners are downsizing to a condo or smaller home, you keeping the home security contract may be a condition of the sale.
You need to move and bring your existing components with you
You might need to take your existing home security components with you to a new place when you move. It should be pretty easy to do, as most alarms and smart devices are portable. You can remove each device by yourself or call a professional installer to come and take down everything. Then an expert will put in the same features -- and any additions you need to make -- in the new place. The home security contract will stay the same.