I suppose you could, but if you're going to go to the trouble of making a 21mm model, the prestwin chassis is far from ideal. You would only end up using the two solebars only,stripped of the hanger detail present, needing a new buffer beam, buffers, coupling hook, brake gear, wagon base and so on.
Getting 3.2mm styrene channel and starting from scratch would produce a far more accurate wagon. I'm sure you can get fold up brass w-irons for 21mm, the rest would just be whitemetal detail.