The difference is probably within the ballast/driver transformer. It may draw more current while attempting to light.

 

SUCCESS!!! I got my new transformer (115vac to 10-0-10 @5A). Installed it in place of the old monster ones. Ran perfectly so I mounted everything up. Final stats: weight cut almost in half (20lbs now vs almost 40); Amps drawn also cut in half (1.4A now vs 2.8A) and no start-up surge of almost 9A before. Half the power draw and half the weight is total success for me. I did sacrifice a bit of light output but totally worth is for a much more usable light. That power surge always got me and would pop a breaker at the worst time. Very happy. Thx for the input as I went thru this guys! Now I've got to order up the parts to do the other one! Pic is of the new led light, heat sink with fan, power supply, and transformer (under power supply) all mounted up.

 

OK, and thanks for sharing the results with us! The reduction is quite a bit! And the heat sink for the LED is quite massive. Now I think of some of the shows I have seen where there is a long row of those lights across the front of the stage and what must be going on in the power and controls areas. Your conversion seems to be worthy of sharing in some suitable publication!!