There have been no indications that Windows Phone 8.x apps will not work on WP9. Would be rather silly if you ask me. I don't think Windows Phone 9 in and of itself will make much of a difference. It's a matter of what platform Dev's want to release on.
Now, with WP8.1, they did make a change in the way the apps are packaged and deployed, but it would not be a major thing to get older apps updated if the dev's put a little time into it.