In researching microwaves I have 2 criteria are needed:
1. The new unit must fit the width, height, and depth of the opening - AND - must have clearance around it inside the cabinet as required by the MFG for a built-in microwave.
2. Finding a microwave where the TRIM KIT is available. The Trim Kit is the vented surround the covers the opening in the cabinet. Many have altered their existing trim kit to fit their new microwave.
I have the Sharp R-820BK convection microwave. The microwave is still available for purchase, but the Trim Kit (RK-82K) is no longer available and is almost non-existent.
