I noticed that the xpj of RH8 shows majorversion 3 and RH9 shows majorversion 4.
After upgrading a project from RH8 to RH9, all it took to open the project back in RH8 was to change the majorversion of the xpj back to 3 and to delete the cpd file.
Then the project opened in RH8 just fine.
But, it will probably not be that simple. I can live with newer features that are ignored by RH8, but not with newer features that crash the project in RH8, or make it look weird.
Any specific caveats to look out for?
Will the project "upgrade" again correctly (meaning, RH9 features that were ignored are not lost when upgrading the project again back to 9)?
(Why? If you want to transfer the project to someone who has only RH8)
Editing project files in this way is certainly not advisable. If you want to continue this approach you can't blame Adobe or anyone else if things go belly up. You may be OK if you have no RH9 functionality at all in your projects though.
For an overview of new and changed features in RoboHelp 9, see Peter's RoboHelp tour: www.grainge.org/pages/authoring/rh_tour/index.htm.
I think the only thing that you really need to avoid in RH9 is the review feature. You may lose other information (snippet categories) when going to RH8 but I think that most will keep working. But as Colum said, it's a gamble and when it goes wrong, you may end up with a corrupted project.