[villagetinker]

OK, the photo attachment showed up this time.
OP, I did not see any attachments, but I have helped a few people replace microwaves. First, they are not all the same size usually the height is different the depth and width need to be the same to fit in the space between the cabinets. The other possible problem is if this is venter outside the vents may not line up between the new Microwave and the existing vent in the wall. Finally, if there are mounting screws in the top cabinet, these may not be in the same location.
I agree the trim, possibly both the side and the front will need to be removed. The microwaves usually tip forward after the front screws in the top cabinet are removed to allow the back of the microwave to be lifted out of the rear mounting strip.
I would strongly suggest getting the same brand and double checking with the store that all of the dimensions on the new one are the same as the old one, this can save a lot of time.

Feel free to send me a PM if you would like to discuss over the phone.