:-\ you can’t do that
motiontweens are for symbols shapetweens are for graphics. if you want to shape tween your text into a shape … put your text on a keyframe, go to modify > break apart then with the letters highlighted go to modify > break apart … again
now your text is just shapes and then 5 or 10 frames later (whatever you want) put another keyframe with the shape you want it to tween into on the frame
then do a shapetween on the first frame of that sequence and it should work. but you can never do a shapetween between two symbols.
hope this helps
good luck 
