This is possible, however, what did you do to start the cycle in the new tank, besides add existing live rock? Also, did you see an ammonia spike, even a small one?
Personally I wouldn't depend upon your LFS and a single test to verify that the tank is cycled.
In almost all cases, you need to add a source of ammonia to really get the cycle started. Usually a raw shrimp from the seafood section of your grocery store is preferred. It is possible that there is enough bacteria on the old rock so that the system didn't show the usual ammonia spike, but you can't be sure unless you add a serious source of ammonia.