The recompose portion should be easy enough to test  and if it doesn't work then just delete the desktop and allow the pool to create a new one based on the Win 8 parent image.   The pool itself isn't tied to a specific OS only the parent image you point it to. 

The only time I can think that wouldn't work would be in the use of persistent disk.    Also I'm not sure of the stance on persona profiles moving back and forth between Win7 and 8.