It doesn't make sense to me that they would force a change of DRM on the ebook stores. It would be a whole lot of hurt, confusion and anger for paying customers. It would only drive more customers to Amazon.
Pure speculation on my part but after seeing on The Digital Reader that Simon & Schuster announced a new library ebook pilot with Overdrive
http://www.the-digital-reader.com/20.../#.UuFngnn0CX0 the scenario that makes sense to me is that they are stipulating that Overdrive has to use the new "secure DRM" to lend the ebooks. The publishers are big on introducing "friction" into library lending so they wouldn't be concerned if there is a whole lot of hurt, confusion and frustration for the library loans.
We'll see if I guessed right.