So, you’ve decided the time has come to upgrade the first layer of your home’s security—the deadbolt lock on your door—to a smart model. The next step is to decide if you should replace the entire lock, or if you should retrofit the one you already have. Hang on, let’s back up a second. What exactly is a smart lock, you ask? It’s any door lock with wireless connectivity and a motor that enables it to be opened without a physical key—and in many cases, instead of one. Smart locks can be operated with some combination of numeric keypad, smartphone app, NFC card, via the internet, with a biometri…