I did this work about 8 years ago. There are square headed screws to the ceiling support and to the side cabinets; some may be hidden under the side panels. If you can't find them, use a sawzall with metal cutting blade. I did not try to save the old cabinet; I made them out of white/red oak. Since the TV cabinet was on an angle, some a small (~8") spacer cabinet was made to extend the side cabinet to mate with the new replacement cabinet. If I had to do it over again, I would probably make a new longer side cabinet. This also relieved a bare triangular portion of the ceiling which I filled with some material from the frig enclosure.
Hope this helps, Rick