I just replaced an under cabinet Spacemaker II that was 16 years old with this model PEM31SFSS. It is 24-inches wide and fits the 24-inch wide cabinet.
My previous model did not use a hanging kit or bracket. Look inside your cabinet, remove the coffee cups and see four 10-32 screws that screw Directly into the microwave top. Loosen those screws and catch the falling microwave.
It is almost a guarantee that the new bolt hole pattern does not match. This is a royal pain to install.
First. At least one or two holes in the cabinet will be close to the new location but not close enough. This required drilling the old hole with a 3/8" drill then cutting 3/8" dowel and gluing it in the hole.
Second. The hole pattern on the microwave threaded holes is not in any units of measure I've ever seen. If GE had half a brain, they would use integral values like 13-1/2 inches between holes, not 51.3 mm.
Third. People may not realize that a 24-inch wide oven requires a 3/16" gap on the left side so that opening the microwave door does not rub against the newly refinished cabinetry. Of course that means the oven will protrude 3/16-inch out of the right side of the 24-inch wide cabinet.
Fourth. The thread type on the 4 microwave unit threaded holes is tight so that one must PRECISELY locate the new holes in the cabinet. You can try using a Tee Square and transferring dimension under the cabinet but that did not work. I finally used a stiff piece of new cardboard as a template. Transferred measurements to it, then pushed small drills through each cardboard location to confirm they engaged the microwave holes.