I have a quilt on my queen size bed that took me about 5 years to make (we moved twice in that time and it has tons of applique so took forever). We have been considering buying a king size bed and I've wondered the same thing. I thought I would just make a 3 sided piece (like a border but for just the sides and the bottom) to add on with snaps. It would be like seeing a bed skirt coming out from somewhere underneath the quilt drop part. I figure if I ever wanted to take that piece off I could use it as a queen size again by just removing the snaps. I do like the idea of taking the binding off and adding then rebinding. It wouldn't work for mine because of the way I bound it but it sounds like a tried and true method. Let us know what worked after you do it ok?