I'd duplicate the shape layer, place it above the one you want to move. Change the bottom layer track matte to Alpha matte, then move the bottom layer. it will retain it's shape as it moves off.
Johnny Cuevas, Editor
"I have not failed 700 times. I have succeeded in proving that those 700 ways will not work. When I have eliminated the ways that will not work, I will find the way that will work."
---THOMAS EDISON on inventing the light bulb.