The spline pattern between the two types is the same, so they didn't change that aspect. The length of the freehub body is the same, the lockrings haven't changed, and the cassette takes the same amount of space on the freehub body.
If you go to 9sp, what are you picking for shifters? Do you have Gripshift, separate Shimano shifter pods, or integrated shift/brake controls? If I were moving to 9sp I would be getting Megarange while I was at it, that would be the primary attraction for me.