Re-Keying is the process of taking the cylinder from an existing lock apart and replacing the combination tumblers with new tumblers set to a different key combination. The advantage of re-keying over replacing is cost and appearance. I am putting the same lock back on the same door, so the appearance will be the same as it was before. It just works with a different key.
When I re-key a lock I don't just re-key it, I perform a complete service on the lock using this 7 step process: