Lengthening HMS Unicorn is possible but expensive

Could HMS Unicorn be rebuilt longer to operate larger aircraft?

Yes, but cutting and lengthening the ship would be a major and expensive reconstruction.

Physical possibility does not settle whether the conversion is wise. Shipyards can insert a new midships section and rebuild the flight-deck arrangement around larger aircraft. That work affects structure, systems, and internal spaces rather than merely adding deck length. A navy must therefore compare the conversion with buying or building a more suitable carrier.