Still nothing from Growwurks but I did send an e-mail to the tech dept at cap controllers (they manufacture Nextgen) and found a reply when I got home tonite.
It would not be advisable to use a 400W bulb on a 600W ballast; they are rated for a reason. And using a low wattage bulb on a higher wattage ballast could potentially damage the bulb or the ballast or both. Hope this is helpful.

That was the reply I received from Greg in Tech support. Now he doesnt mention exactly why this cant be done but I cant afford $212 +shipping to see if he is wrong. Maybe there is some difference in the way an electronic ballast works compared to magnetic?? :wtf:
Hell I dont know ....... I'm gonna fire up half dozen bongs or so and go back to E-bay and find the store in Canada that has 600w MH conversion bulbs for $30 :stoned: