I have a kitchen configuration with an existing countertop microwave built into a cabinet above an in wall oven. The microwave I'd like to replace it with (Whirlpool Gold) happens to be an over the range model. Is it OK if I use an over the range model in a cabinet? Reading the installation instructions it's hard to tell since they talk about ventilation, but I'm guessing that ventilation is for the stove below (which won't exist here).
In almost all cases you can do this. The difference between the microwave models is more about the mounting and how the front looks. There are basically three general types. Your typical countertop microwave which some people just stick in cabinets - which is fine given that the electrical is done right. The you have your over the range which usually have two extra functions - an undercarriage with range venting and lighting, and simple mounting brackets on the top. Then you have your cabinet or built-in models. These are the least sold and usually cost a bit more because they have a nice mounting plate in front.
Any ventilation space that you currently have should be adequate for whatever you get. In your particular case my "problem" would be the undercarriage. These microwaves are generally not made for sitting on their bottoms which have a fan and lights and usually a plastic casing. This casing may break or crack over time. I don't see an "issue" with doing this - although I would simply double check by calling Whirlpool - but I wouldn't do this unless I was 100% certain that the bottom of that microwave wouldn't be damaged because of movement and rubbing. Also I can't see how this aesthetically looks the best of the three options.
Microwaves do require ventilation - there are typically cooling fans and airflow is required for them. Many "countertop" models include specifications for required ventilation when mounted in a cabinet.
If the "over the range" model you are considering also has a "range hood" function that might call for more ventilation, but even if not using that function some ventilation is required. I would suggest calling the manufacturer's technical support line and asking questions - I'd also suggest at least considering another model more suited to your actual use scheme, if it is one that includes a "range hood" function that would be of no use in your desired location.
We just did this with a GE Artistry microwave we had over our daughter's range. We bought the GE Artistry fan/hood to install over the range. We used the Ikea Sektion cabinets. We installed a 36" wide "over the fridge" cabinet and installed the plug inside the cabinet. (cut a hole for the plug to be threaded through bottom of cabinet) We then mounted 2 x 4's on the wall under the cabinet and then the bracket that came with the microwave. (this was done since the Sektion cabinets are 15" deep) The microwave was installed to the bottom of the top cabinet, just like you would if it were over the range. We then customized another 36" "over the fridge" cabinet around the microwave. We placed the 30" wide microwave to one side and left a small space on one side for cook books. We cut out squares on the bottom of the second cabinet for the fan and the light just in case someone turned them on. We put a piece of trim just under the microwave and on bottom of cabinet so you cannot see under the microwave in the cabinet. The door sticks out just beyond the 15" deep cabinet. We used Ikea side panels to cover the seam of the two cabinets. Other side had the wall covering the cabinets. I called GE about the installation and she said this was a fine way to install the microwave, although they did not recommend alterations to their directions in installation booklet. The microwave has air circulation when installed this way. It looks very nice. I don't think this could have been done with a 12" deep cabinet. It would not be deep enough for microwave.