We have a 1988 Excella 29 and just removed our corner inserts. Carefully pry off the frame around the fabric inserts. Ours are oak and they came off very easily. Our oak needs refinished and only had a few small finishing nails holding them. The padded fabric insert is a separate piece and it is fabric stretched over a foam pad with a single button in the center to luck like tufting. We are trying to decide whether to replace with new, matching fabric (to the sofa) or use oak plywood finished like the cabinetry. We will not make the final decision until it is time to put the interior back together
Also check the back bedroom if there is any overhead cabinets with an end. There may be another corner insert.