Generally, the scenery tool code is not part of the X-Plane code. There aren’t version 9 and version 8 scenery tools. There will always be one set of scenery tools that let you optionally use the v9 features or not.
An example is the AC3D plugin. Everyone can use the new plugin (and a few small features apply to X-Plane 8 too, like a less sluggish animation UI when editing large objects). If you use key frames, your object will only work with v9 – if you don’t, it will work with v8.
More specifically on key frames: a traditional v8 animated object is really just a key framed object with only two key-frames. So the UI has changed very little – basically where you only had two key frames (points of correspondence between the dataref and animation) in the old plugin, you can now use the “add” button to add more.
I will try to add features to the scenery tools to warn when v9 features are being used, so that if you want your scenery to run on v8 and v9, you can check for v9-only feature usage.