I switched from disposable to washable pads (by Little Bit--I highly recommend them) and had no trouble. I keep their potty area in the corner and for a few days I put a disposable on top of the washable and then removed it. They had no problems going on them. Hallie is 6 and Simon is 4 and they have been pad trained forever.
We even have a social group about them
http://www.yorkietalk.com/forums/group.php?groupid=10