From Daniel Spreadbury, Feature & Documentation Manager at Sibelius, I received some very good news about version backwards-compatability in the new Sibelius 3, in contradiction to what I had said about Sibelius 2:
Sibelius 3 is able to save files in a format that allows them to be opened by Sibelius 2 (and, of course, Sibelius 3 can open files from all previous versions of Sibelius). There were too many radical file format changes between Sibelius 1.x and 2.x to make it possible to retain backwards-compatibility when we were working on Sibelius 2, but we took notice of user feedback following its release and took special steps to ensure we would be able to save as Sibelius 2 in Sibelius 3.
Contrary to what those in the industry might assume, this actually makes me willing to upgrade sooner rather than later; I was prepared to wait until the prevalence of version 3 made it absolutely necessary.