Q: Is it cheaper to rekey or replace locks?
A: Generally speaking, it is much cheaper to rekey locks than to replace them.
The process of rekeying locks basically means changing the pins inside the lock cylinder as shown below:
In other words, you just have to change the pins to have a new key to operate the lock.
Replacing a lock means to completely change all the lock parts, including new cylinder, latch, housing, etc as shown below:

Therefore, changing a lock is more expensive. Sometimes it could cost up to 3 or 4 times more than rekeying the lock.
Finally, always ask to rekey your locks instead of changing them, especially if there is nothing wrong with the locks.
Q: Is it possible to rekey high-security locks cylinders as well? I know that these locks have a unique keyway and therefore it may be a problem to rekey them.
A: Rekeying high-security cylinders is possible, however, not any locksmith or shop can do that. In addition, it requires the locksmith to have a unique rekey kit specifically for the same brand as the high-security locks that need to be rekeyed. Therefore, it is much less common. We would highly recommend calling around to make sure the locksmith has the necessary tools and kit to do that.
