Are you moving the image in vcenter or anything? Personally after suffering issues with that even in 6 versions I never change anything on the vcenter side thats in use, it uses a little extra storage but I clone the parent image to seperate ones per pool, and if I need to make a change I clone that and use that for the pool and delete the older one. That way your unlikely to run into these issues, even though its was supposed to be "fixed"