These types of conversions are pretty common place and I get several requests for this each year. There's no debate about lapstrake, regardless of type being lighter than carvel as well as other methods, such as batten seam, double planked, etc. With the exception of molded veneer builds and of course the introduction of manmade fabrics, lapstrake is the lightest of the traditional builds and glue lapstrake much lighter.
Plywood as planking is actually weaker than solid wood planking, but being much more dimensionally stable and coupled with glued seams, permits the same thickness planking, while the gross reduction of other structural elements, (no frames, no stringers, etc.). In fact, it's generally accepted that you should use the same thickness plywood as the original planking was in a carvel build. This is because of the weaker longitudinal stiffness of plywood, compaired to solid stock. In a traditional build comparison, keeping things relatively equal, (solid stock planks, typical frame arrangements and spacing), there's little difference, in fact possible a slight disadvantage to lapstrake. On the other hand, if you take the longitudinal stiffness the laps bring to the mix, you can decrease the size of the framing elements and the planking slightly, allowing the hull shell bare more of the loading. This is usually accomplished by increasing frame spacing, though can (has) be carried out to extremes on some racers.
Lastly, given a choice, plywood planking is simply a better material than solid, for several reasons. It is heavier than solid of the same physical dimensions and isn't as stiff longitudinally as solid, but cross grain stiffness and strength is much higher, plus the dimensional stability and if coupled with glued laps, makes the end result considerably lighter. Of course typical plywood is quite ugly, compared to solid, but sliced veneer plywood is available if you need the bright finish.
