Our Challenger was supposed to have a Whirlpool convection microwave, but was shipped with a standard one. Thor shipped us a convection to replace it, but initially wasn't sure they had one in stock. They were going to send us a check to purchase one on our own, so we checked the model number and found the exact model on sale at Home Depot. The mounting was exactly the same as the standard microwave. It was the same width and height, but the depth was a little longer. However, it fit perfectly without any modifications.
You might check the manufacturer web site to see if they offer a similar model as you have in a convection. It took us a few hours to swap it out, but are very pleased with the result. Good luck.