You moved into a house with a pre-existing security system
Sometimes when you purchase a home, a home security system is already established. All the components function well, are placed in the right places, and are ready to go. In this example, you might find that you have to call and establish a new service contract. A Vivint installer will still come out to make sure there are no compatibility issues, and they may offer ideas on upgrades. It's a quick appointment, and you can get your 24/7 monitoring set up without worrying about any new components -- unless you add on.
The old owners can transfer an existing Vivint contract to you for a nominal set-up fee to make the process simpler. If the owners downsize to an apartment complex or smaller home, transferring the security contract might be a part of the sale.
You want to move and take your existing components with you
You may want to take your old smart devices with you to a new house when you move. It should be fairly simple to transfer, as modern alarms and smart devices are portable. You can take down each alarm or camera by yourself or contact a professional installer to come and take down the system. Then a pro will hang the same features -- and any additions you want to have -- in the new place. Your home security contract will remain the same.