Well, we're gonna be picking up my 2010 G37 at the end of the week, but I'd like to know ahead of time whether the spare ballasts I have (35W) would work for converting the fogs to Xenon/HID. Or do I need to buy 55W ballasts and bulbs?

 

you can always go from 55W to 35W with no problems. where you will have problems is going from 35W to 55W.

so short answer, yes you will be fine.

 

I converted my fogs to HID 35W ballast and it works fine. Only comment is the fog light housing is fairly broad so the light scatters a lot more than the main headlight projector. I installed 6K Xenons and they are bright for oncoming traffic.