If you are using Shimano shifters, you will need a Shiftmate to make the shifting work properly. The 2 systems are generally incompatible due to different actuation ratios. What do you hope to attain by changing the RD? Complicating what should be a simple system is not an upgrade, and other than shaving a few grams or adding the bling of a Ti RD, there is really no functional upgrade in doing this.