You moved into a place with a pre-installed security system
Sometimes when people close on a house, a security system is already established. All the alarms and cameras function well, are placed in the right areas, and are ready to go. In this example, you might find that you have to call and start a new monitoring contract. A Vivint installer will still come out to ensure there are no integration issues, and they might offer ideas on where you should replace bad outdated components. It's a preliminary appointment, and you can have your monitoring set up without having a bill for any new alarms and cameras -- unless you make additions.
The previous homeowners can transfer an existing Vivint contract to you for a one-time transfer cost to make the process simpler. If the previous owners are moving to an apartment or smaller home, turning over the home security contract might be a part of the sale.
You are looking to move and take your existing equipment with you
You might need to bring along your old home security components with you to a new place when you move. It should be fairly simple to accomplish, as many components are plug and play. You can uninstall each device by yourself or use a professional installer to dismantle the system. Then a pro will hang the same devices -- and any additions you want to have -- in the new place. The home security contract will remain the same.