Abstract: An attachment (9) which transforms a lock cylinder having a cylinder housing (30), an aligning bracket (27), and a cylinder core 41a into a lock cylinder with an interchangeable core (40a). The interchangeable core (40a) is held in place and is attached to the interchangeable attachment (9) using a conventional cylinder core nut (21). With a control key (50) inserted into the cylinder keyway (42) within an interchangeable core (40a), a push rod (51) comes into contact with a plunger head (19) at the front end of the plunger (11). The plunger (11) is pushed back, allowing the locking balls (13) to recede into the inward position; at this point, the only thing holding the interchangeable core lock cylinder (40a) in the cylinder housing (30) is the security back-up pin (14). With the control key (50) inserted, turn right five degrees and pull. The interchangeable core lock cylinder (40a) will come out. A new interchangeable core lock cylinder (40a) can be inserted using the same procedure.