I did it once. The quilt was too cute to pass up. I had to remove all the quilting and remove the borders because the borders were cut too short. The quilt bowled in the middle because of that. I also had to replace the backing because I put a few holes in it with my seam ripper. Then I added batting, sandwiched it and quilted it.
It took a long time and a lot of work. I probably won't do it again, but it was fun to resurrect it from the junk heap.