held in a locked positioned in magazine 10 for changing the thread needle bar 4 is stopped in its upper dead center position ot compressed air cylinder 51 is operated so that its forkhead 52 extends in an upward movement the movement of forkhead 52 has no effect on rocker arm 6 until pin 54 carried by its bears on the bottom surface boundary of oblong hole 53 provided in forkhead 52 in the further course of the upward movement of forkhead 52 receives a switching stroke adjoining its upper dead center position and extending from ot to os needle holder 13 is thus retracted into magazine 10 and its lateral surfaces 19 20 are conducted to inner walls 21 22 of magazine 10 at the same time recess 24 moves into the vicinity of guide 23 so that the relative position of needle bar 13 to be disengaged is secured in magazine 10 during this upward movement of the needle bar upper bevel 42 of catch 41 of retaining clip 31 engages and bears on lower bevel 46 of holding jack 44 associated with needle bar 13 to be disengaged and imparts to holding jack 44 a swivel movement about its shaft 47 opposite to the biasing force of spring 49 as soon as catch 41 is above holding jack fig3 the free end 37 of horizontal arm 32 bears on stop surface 38 of magazine 10 when needle bar 4 continues its upward movement retaining clip 31 performs a clockwise swivel movement so that boundary edge 37 of recess 34 moves away from needle bar 4 and the needle bar 4 is thus released from the needle holder 13 as soon as the frictional connection.