Scenery based on DSFs, OBJs, and other version 8/9 file formats should work with X-Plane 10 unmodified.This includes orthophoto scenery based on DSFs - we're not throwing that code out.
The new rendering engine features for version 10 (and there are a lot of them) are extensions - new ways to render things, new types of art assets.
I do believe that we may drop support for ENV scenery files in version 10. We've had DSF for six years now, and ENV's capabilities (a 500m mesh, very limited orthophoto resolution) aren't useful to today's users. You can use DSF2Text/XGrinder to extract custom object placements from an ENV for use in a new overlay.
We may also drop support for OBJ version 2. (Yes, we still load OBJs version 2.) OBJ version 2 is the OBJ file format from X-Plane 6, the one before OBJ 7. If you have any old OBJs (version 2 or 700) you can use XGrinder to automatically batch convert them to OBJ8.