Was stunningly easy. Due to cracks near three screws on end near handle, the existing outer shell flexed easily and popped off. I had just bought hard plastic automotive door panel tools at Harbor Freight for $10 and they were perfect for this application.
Two things: you do not have to remove the inner shell but you do have to watch for the existing glass to be loose and could slip out should you ignore it. The glass is attached on the new part and used to be on the old one but it came loose during removal. Have to watch this.
Transfer old handle to new frame (two screws) easy. Due to old frame cracks the lower end of the handle was popping out about 1/4". When transferring the handle to new frame I found that the lower screw had stripped out the hole on the handle and the small plastic pieces fell out. I tried a slightly larger self tapping screw but it also broke the plastic. Found a smaller but slightly longer screw that worked by going into the unused part of the screw base in the handle. There is not much pressure on handle so it is tight and secure as is but if it loosens in the future I will order a new handle and install. I didn't want to order a new handle and wait to finish repairs for 2-3 days so am trying this.
In retrospect though, with the handle loose, I should have just ordered a new one which will last the life of this microwave oven.
Bottom line: this was surprisingly easy job.