Quickly rekeyable lock cylinder and plug assembly thereof

A quickly rekeyable lock cylinder comprising:

• a cylinder body;
• and
• a plug assembly disposed within the cylinder body comprising:
• a plug body having a longitudinal axis, a transverse axis vertical to the longitudinal axis, a keyway and a rekeying tool opening;
• a first rack group disposed within the plug body having a plurality of first rack components which are biased and located across the keyway;
• and
• a second rack group disposed within the plug body having a plurality of second rack components in engagement with the first rack components and at least one tool-receiving portion, wherein the rekeying tool opening exposes the at least one tool-receiving portion of the second rack group and the second rack components are capable of being moved along the transverse axis of the plug body while being restrained against movement along a longitudinal direction of the plug body for releasing engagement with the first rack components.