Both rekeying and replacing locks stop old keys from working. The difference is cost, and whether the hardware itself needs to change.
Rekeying
A technician changes the pins inside the existing cylinder to match a new key. The lock body and handle stay. It is usually the cheaper choice and can put several doors on one key.
Choose rekeying when: the locks work well; you have moved in; keys are lost; a tenant, roommate or contractor had access.
Replacing
The whole lock or cylinder is swapped for new hardware.
Choose replacement when: the lock is worn, stiff or damaged; it is low grade on an exterior door; you want a different function such as a keypad, smart lock or high-security cylinder; or the lock was damaged in a break-in.
Often, it is both
Many homes need a mix: rekey the good locks, replace the one worn side-door lock, and put everything on one key.
How to decide in five minutes
If the lock turns smoothly, the finish is sound and the hardware is a decent grade, rekeying is usually the right choice. If the lock is loose, corroded, hard to turn or an obviously low grade, replace it.
Combining both
Sometimes the best approach is to replace the exterior deadbolt and rekey the rest so the entire home works on one key. Ask the locksmith to explain the difference in cost and outcome.
After the work
Test every key, check the strike alignment and record how many keys were made. See new home rekeying and hidden costs of cheap locks.
Situations we see often
A tenant moves into a rental with good-quality deadbolts: rekey. A homeowner finds that the front door key sticks and the latch is loose: replace. A landlord turns over several units: rekey each and keep a register. A business owner wants restricted keys: replace with a restricted cylinder. Asking the technician to explain why they recommend one over the other is always reasonable.
For rekeying and lock changes in Toronto, see lock rekeying.