Peter, not sure morph would do this too well. I would just get a hand moving in paper and adding it on top as think that would work a lot better. I've mocked up an example for you, it's only a quick proof of concept and with more tweaks sure you could produce better. All the images are from the free library as well.
Oh hang on, just re-read that and you want it the other way round!
So as you cannot move the paper out again you would use morph (remove colour as this does not morph).
Again see attached
Peter Love
I want to morph two images - a full paper tray and an empty one and animate it to make it look like the amount of paper is reducing from full tray to empty tray.
Is this possible? How can I do it?