To provide lock servicing of particular door locks in common use, a bypass key (1) with key blade (2) long enough to reach through a lock-cylinder plug (26 and 45) and low enough to pass under extended tumblers (37) of a lock cylinder (28 and 43) in order to dislodge a tailpiece (36) from a stop-washer (63 and 45) or other attachment means at an inside end of the plug is provided in conjunction with a control key (14 and 21) with a key blade (13) of the same length into which single-lock and multiple-lock profiles (15) can be made to rotate the lock plug (26 and 45) and disengage or engage a lock cylinder (28 and 43) and its means of attachment to a door lock (54). Methods are provided for using the bypass keys (1, 1A, 18, 22 and 23) in conjunction with the control key (14 and 21) for unlocking a door (55) and installing a new or changed lock cylinder (28 and 43) without removing the lock (54) from the door (55), a time-consuming process that is required in current lock-change methods, regardless of whether or not a key for the lock is available.