A key-operated cylinder lock for operating a bolt or a latch, that can be programmed for use with one of a plurality of user keys without disassembling the lock or replacing the tumblers, with reduction or elimination of incidental or accidental re-keying of the lock. The lock has a rotating plug having one or more retainer cavities formed into the periphery, and lock configuration change balls, movable within the lock between a first position within a driver chamber and a second position within a corresponding retainer cavity when the plug is in a programming position. The positioning of the change balls within either the pin chambers or the retainer cavities determines the key configuration that can operate the lock. The lock employs a means for isolating selectively the retainer cavities from the corresponding driver chambers when the plug is in the programming position, to prevent incidental or accidental movement of the change members from the driver chamber into the retainer cavities.