I use 6.5 still too... when I was using the tryout of 7, I had a bit of success copying/pasting from one to the other, but of course, that doesn't mean everything, so you're basically rebuilding the project. I had downloaded a few AEPs from links here that were for 7.0, and converted a few to 6.5. If I actually made money, instead of being a hobbyist, I'd probably upgrade... I like several of the new features, and the new interface, but it's hard to justify in my case.
I haven't spent much time, but you may want to create some simple projects in both version (that are identical), then open them in a text editor (a smart one, not "word pad" or "text edit") that can point out the differences. I've made many older apps open new files after I Discovered the differences, so that might work (a copy, of the project, of course...).
I'm no top hacker, and my guess is that anything really complex is not going to work, regardless. This topic came up before, right when AE7 came out, so you might search.
I think it's been said, but really, if you're going to use it, "it's time to update your application."