Well, whether or not the CG would change depends on the type of retract, or rather the direction of retraction.
If the retracts move in towards the fuse (or out), then the CG won't change, but if the retracts rotate and move forward (or back) then the CG will change.

Mike,
Ballons and squeeze bags? Why not lead shot in a tilting box or tube? (With hatches to prevent unwanted changes of course.)
Dennis-