You need the longer air barrel, the adjustment screw, and the clip that keeps the screw inside the barrel, all from the G250V. I don't have a G250V schematic so I don't know the current part numbers. I do know that the G250V air barrel fits the G200B case. I was considering doing exactly this because I have a NOS G250 adjuster, and would only need to buy the air barrel and clip. Those parts were over $50 at my LDS, so I bailed on the idea. I bet the adjusting screw is pricey.
I can't tell any difference in performance between my G250 (old style) and my G200B, when both regs are tuned well. In fact, the G200B is a little lighter, which might make a difference to some people. The nice thing about the G250 is the fact that you can adjust out slight free flows as the seat wears, which means you can set it lighter when rebuilding. But I'd be reluctant to spend $75-80 in parts to convert one, unless the G200B air barrel was already corroded or needed replacement.
