I did this when I got fitted @ Manchester Lane.  

I currently play the 909 D1, and I disliked it.  I hit it really high, to the point where I felt the ball was ballooning and losing distance on me.  It also created too much spin.  (Sometimes I was even getting backspin on the driver.)  I also found it difficult to control a fade or draw, and my misses were big time misses.

I hit both under the same setting, same loft, with the same shaft (ALDILA NV 55 Stiff).  I liked the 909D3 better.  First off I loved the look and the feel when I swung it.  It also had a nice crisp 'ting' sound, vs a tin can like 'thunk'.  I could work the ball much better with the D3 than the D2, plus my misses were less magnified.  My good shots were dead on, the trajectory was ideal, and I spun the ball much less with the D3. 

Carry distance was about the same, but total distance was much better with the D3.  I really noticed the difference in the trajectory, workability, and the lack of spin.

I would suggest getting fitted.  It was really fun, and I learned a lot.  The shaft made all the difference in the word for me.  Once I found a shaft that felt good and gave me good numbers, picking the head was easy.  If you can't get fitted and you like the D3 I would say stick with it.