Desperate need help for upgrade light fixture

[robert J]

I have a 36" Aqualight Pro with 150W metal halide. I was posting up for sale about a week ago, but now I want to keep and will upgrade it instead. But I have no clue how can I change from 150W to 250W ? I know it needs to change the ballast ... but how about the rest ? Do I need to change to a new DE socket or can I just change the bulb ? How about wiring ?
Pleaaase help ! I have no idea what to do with this fixture.
I woulb be thankful for any advice.