I've bought Motorola Droid Razr M from eBay. It is locked with Verizon by default, so seller unlocked device for me and it is working fine. It comes with Jelly bean 4.1.2 and I found that Kitkat update for this device. Now the question is Will my device get locked again after update from 4.1.2 to 4.4.2???
There's a 50-50 possibility of getting the phone locked because if the seller has unlocked through the proper way(legally) then you can go ahead and update your O.S else if your seller has unlocked by other means then definitely the phone will get locked to the Carrier with which it has the contract with(here Verizon).
I suggest you to get in contact with your device supplier and ask him his mode of unlock. An unlock never comes free of cost although some sites offer a legal unlock .The Same thing is what observed in other mobile OS(like ios) if you have unlocked it by other means and have updated your device then the phone gets locked to the carrier with which it was sold and you need to unlock it again.