i think once you have made the move, kinda hard to work backwards KWIM? i def wouldn't threat unless you will follow thru. maybe don't say anything and see what happens in the next day, then just put the crib(cot) back up and say "if you can't stay in your big boy bed, you need to be in the little boy bed" and see what happens? I'm assuming the cot is still put together and not being used?
i don't like the idea of a lock either but if you need to keep him in his room for safety reasons, i think it is fine. i do have a friend that uses it bc her little boy is very spirited and he would walk out of the house if he could in the middle of the night...so yea, they need it for safety reasons!
also, when we were dealing with all this (sooner than i planned bc ds was climbing out of his crib at night), i eventually made a sleep chart w/ sleep rules and also used a clock that used colors to indicate when was wake time vs sleep time vs nap time.