Rekeying sounds technical, but the concept is simple: change the internal pins so old keys no longer work, then cut new keys for the new pin configuration. Here's an inside look at exactly how it's done.
Most residential locks use pin tumbler cylinders. Inside are 5–6 spring-loaded pin stacks, each consisting of a key pin (bottom) and driver pin (top). The correct key raises each pin stack to the exact height that aligns the gap (shear line) between key pin and driver pin, allowing the cylinder to rotate.
The locksmith removes the cylinder from the lock, uses a follower tool to disassemble it, and replaces the key pins with different size pins. The new key cut pattern corresponds to the new pin heights. The hardware, cylinder shell, and driver pins stay the same.
The lock body, cylinder shell, springs, driver pins, and all hardware remain identical. The only change is the key pins inside the plug. This is why rekeying is significantly cheaper than full replacement — most of the lock is reused.
Rekeying requires that replacement pins be available for your lock's brand and model. Most major brands (Kwikset, Schlage, Defiant, Baldwin) have widely available pin kits. Some exotic or very old lock brands may not have replacement pins available, requiring full replacement.