Removing an OTR microwave is a matter of removing a few screws. Two screws removed from the cabinet above will have the front of the unit falling into your hands. The rear is held by a backing plate. There may or may not be a latching mechanism holding the rear, but usually you just pull the OTR loose from it. Then removed the backing plate from the wall.

Usually the hole from the vent, if it actually vents to the outside, is rectangular 3.5x10". You'll need to find a vent that matches. Vents are usually hardwired, microwaves are not. Hence you would need to make a pigtail for the new vent to reuse the recepticle that the microwave was using.