Rekeying is usually the cheapest way to stop old keys from working, since it reuses your current locks instead of buying new ones.
Because the technician re-pins the cylinder you already own, you pay mostly for labor and a few small parts rather than a whole new lockset. Doing several doors in one visit lowers the per-door cost further.
When your hardware is still fine and you only need to invalidate old copies, rekeying delivers the same security reset as a full change at a fraction of the price.
Call for help: (888) 793-3933 — a straight answer and an honest estimate before any work begins.