Hey Walter,
Im not sure if your ? Has been answered but I have the same light fixture as you only I have the 24" for my 60gal cube. I thought about doing the upgrade myself about 2 weeks ago but until I finally called Coralife ( by the way #is 1 (800) 255-4527) and was told that it is possible, but too much work and parts that may even be too difficult to find will be needed in doing this upgrade. I don't remember the exact parts but I do remember been told that obviously the ballast would need to be replaced along with the exit cord comming from the fixture itself due to the 250watt ballast have a much thicker round connection also the housing of the bulbs are different, and ofcourse the bulb itself. Oh and by the way they don't have or should I say make a upgrade kit, you will need to get all this yourself. Well hope I was able to help. Good luck.