If the bow is tuned you don't move the rest. Here's what I do when swapping sights. On the sight I am removing I measure from the mounting surface to the pins. Then if I have a stabilizer I measure from the top edge of the stabilizer to the height of the 20 yard pin. Assuming the new sight sticks out the same distance (or close) from the front of the bow then you can transfer these measurments to the new sight adter it is mounted. This should get things close enough to get you started resighting the new one.
If You're Not Living on the Edge You're Taking Up Too Much Space
Martin/Rytera Staff Shooter
PSAA Life member, UBP Life member
PADI AOW Diver