You moved into a house with a pre-installed security system
Sometimes when you buy a house, a home security system is already established. All the alarms and cameras work, are installed in the right areas, and are ready to go. In this example, you might find that you have to call and set up a new monitoring contract. A Vivint installer will still come out to ensure there are no compatibility problems, and they could offer ideas on upgrades. It's a quick appointment, and you can get your 24/7 monitoring set up without worrying about any new alarms and cameras -- unless you add on.
The previous homeowners can give an existing Vivint contract to you for a nominal transfer cost to make your life easier. If the owners are moving to a condo or retirement community, turning over the security service may be an addendum of the sale.
You need to move and bring your home security equipment with you
You may need to bring your old home security components with you to your new house when you move. It should be fairly easy to accomplish, as many smart devices are wireless. You can uninstall each alarm or camera by yourself or call a professional expert to come and take care of the system. Then a pro will put in the same devices -- and any additions you need to make -- in your new house. And your home security contract will remain the same.