I recently stripped the backing off of a quilt after I'd glued it on with Elmer's. I noticed it was already starting to pill up so it had to go. I just soaked the quilt in my tub for about 20 minutes (lukewarm water) and then the backing peeled right off! I kept the batting attached to the top - had to re-smooth parts of it when it came out of the water but I let it dry as-is and enough glue stayed to keep those two parts together. (I laid it as flat as possible in the tub and let it air-dry there.) Glued on a new back and I was good to go!
It's pretty forgiving glue - it's never "permanent", not even when you dry it with the iron.