I've also done the same - switching between two 4G capable phones that I own. One of them, I received last year when I activated my Verizon account (came back from Sprint), and the other I purchased a couple months ago at full retail price. The SIM cards in my case, aren't the same size so I can't swap the cards but what I can do is reactivate the SIM using the Verizon online system. Go to My Verizon, Activate or Switch Device. You need the IMEI Number and SIM Card number of the unused phone and you can activate it yourself. Doing so with the power off, then turning the power on after you enter the numbers, only takes a minutes or so while the phone is powering on.
But, if two phones have the same SIM and you simply want to swap them, as long as the SIM cards are the same size, you can swap the cards. I also did this between my husband's and my Thunderbolt phones when his started acting flaky. Mine was fine so I swapped cards, powered both phones ON and they were equally able to use their own data plan.
What a nice convenience for those of us who don't mind a change in our devices from time to time.