kahit di na siguro yung keyless entry kasi unique naman ang frequency per user eh and AFAIK mahirap magpaduplicate nyan unless you go to the manufacturer of the system itself...
i'm not sure on the cost of changing locks and keys though..
If the car uses an ignition with a transponder in the OEM key (i.e. BMW), they can deactivate the old codes and program a new code so that even if the key can fit and open the door, it cant start the car.
Hassle lang with the transponder chip equipped keys is that they are more costly to replace.