The short answer
Rekey when the lock still works fine and you just want old keys to stop working. Replace when the lock is worn out, broken, low quality, or you want a different type of lock entirely.
Rekeying changes the small pins inside the existing lock so it accepts a new key and the old keys no longer turn it. The hardware stays on your door. Replacing means taking the old lock off and putting new hardware on. Both stop the old keys from working. The difference is what you do with the lock you already have.
If you're standing at your door right now trying to decide, ask yourself one thing: is there anything wrong with the lock itself? If no, rekey. If yes, replace.
What rekeying actually is (and what it isn't)
Inside most pin-tumbler locks is a row of tiny spring-loaded pins. Your key lifts those pins to exactly the right heights so the lock turns. Rekeying swaps those pins for a different set, cut to match a new key. After that, only the new key works and every old copy is dead.
Rekeying does not make a cheap lock stronger. It's the same lock with a different key. If your deadbolt was flimsy before, it's still flimsy after. Rekeying is about who has access, not about how tough the hardware is.
One handy thing most people don't know: a locksmith can rekey several different locks to all open with one single key. If you've got a deadbolt, a knob, and a side-gate lock that each take a different key, they can usually all be set to the same key in one visit. That's far cheaper than buying matching new hardware.
When to rekey
Rekey if you just moved into a house or condo. You have no idea how many copies of the old keys are floating around with the previous owner, their kids, the realtor, the cleaner, or the contractor. The locks are usually fine. You just want the old keys to stop working. This is the most common reason people rekey, and it's the smart move on move-in day.
Rekey after a roommate, tenant, ex, or employee leaves and you didn't get every key back. Rekey if you lost a key and you're not sure where it ended up. Rekey when you want several doors to share one key. In all of these, the lock is doing its job. The problem is the keys, and rekeying solves the key problem for less money.
Rekey to save money in general. It's almost always cheaper than buying new hardware, especially across multiple doors, because you're paying for a quick service instead of new locks.
When to replace
Replace if the lock is physically failing. Sticky, grinding, hard to turn, the key catches, the deadbolt doesn't throw all the way, the knob is loose and wobbly. Worn internal parts don't get fixed by rekeying. New hardware does.
Replace if the lock was kicked, drilled, pried, or damaged in a break-in or attempted one. Replace builder-grade locks that came with a new construction home if you want real security. Those bottom-tier locks are easy to defeat, and rekeying a weak lock just gives you a weak lock with a fresh key.
Replace when you want something the old lock can't do: a keypad or smart lock, a higher-security cylinder that resists picking and bumping, a heavier-duty commercial-grade deadbolt, or simply a finish that matches a remodel. And replace if you've lost the only key to a lock that can't be picked open cheaply, since at that point a new lock can cost about the same as the labor to defeat the old one.
Cost, security, and the practical trade-offs
Money: rekeying is usually the cheaper path because you keep your hardware and pay mostly for labor. Replacing costs more because you're buying the locks plus the install. The more doors you have, the bigger the gap, which is why landlords and offices with lots of doors lean toward rekeying between tenants.
Security: this is the part people get backwards. Rekeying does not upgrade security, it only changes access. If your goal is a tougher door, replacing with a quality lock is the only move that helps. If your goal is to cut off old keys, rekeying does that just as well as replacing for less.
A good rule of thumb: change access, rekey; change hardware, replace. And you don't have to pick one answer for the whole house. A common setup is to replace the weak front door lock with something solid, then rekey the rest of the doors to match that new key, so the whole place opens with one key without paying to replace every lock.
Both jobs are quick for a pro, usually well under an hour for a typical home. If you're not sure which way to go, a locksmith can look at the lock and tell you straight whether it's worth keeping.
Need a hand in Ventura County?
CLS Locksmith is a mobile locksmith based in Thousand Oaks, and we cover the Conejo Valley, all of Ventura County, and into the west San Fernando Valley. We do residential and commercial rekeys, lock replacements, safes, and same-day help during business hours.
We can rekey your locks to a single key, swap out worn or builder-grade hardware, and tell you honestly which one your door actually needs. Licensed and insured, CA BSIS #LCO8562, with free estimates. Give us a call at (818) 454-1047 and we'll come to you.