if you want to change all movieclip instances, go to the movieclip's timeline and shape tween there.
You can't use Shape Tween with the Movie Clips, It's not allowed. You must use shapes (that includes merge drawing, object drawing or primitives) to create Shape Tween.
In your case, since you have converted the shape to movie clip, you can create motion tween to reshape. Try with it, if it works. Best option would be to create shape tween with your original shape (no movie clips).
if you follow my directions and go to the movieclip's timeline you'll find a shape which you can tween.