Changing the designed rig from fractional to masthead would probably not be a good idea. It would be expensive to get and install a new forestay, add a new sheave and buy a new jib. It might very well mess up the balance of the boat too, making it harder to handle and not necessarily faster. It would certainly reduce any resale value. Van de Stadt is known for well-designed and built boats.
To improve downwind speed some people do rig a masthead spinnaker on their fractionally-rigged boats, which is much simpler to do. You would need to rivet or bolt a pulley at the top of the mast, run a spinnaker halyard through it and find a suitably sized spinnaker to hoist on it. The mast might not be up to the force of the new sail, however; a strong breeze might turn the mast into a pretzel. We had a fractionally rigged J/36 and contemplated this, since some of them in Oregon have done it. Our smaller, more managable spinnaker allowed us to pass a J/35 on a downwind leg in a windy race, however, while they broached, so we decided not to change.