A longer landing gear to get the plane higher off the ground doesn't fit into the existing gear well in the wings. So you would have to redesign the wings. This

A longer landing gear to get the plane higher off the ground doesn't fit into the existing gear well in the wings. So you would have to redesign the wings. This required making sure that the loading on the main body stays the same. By now you are doing more than 50% the design work of a new airplane and you definitely can not keep the type rating. And if you need a new type rating anyway, you would redesign the cockpit, probably to be similar to the glass cockpits in the 777s.