You can build out a new Windows 8 parent machine and then recompose those pools to Windows 8.   I think that would work in all cases except maybe persistent disk.    I seem to remember they had to be attached to the OS they were created on. 

Is it possible to recompose Win7 pool to Win8? I thought recomposition works within the same OS only.

  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.