Normally the process is a one way street. Once you open the project in the newer version there isn't an easy way to go backward. I'm not saying it can't happen or be done, just that RoboHelp expects that once you touch it and upgrade it to 10, it will refuse to open in 9 because the project now has version 10 identifiers.
Try deleting the CPD file and see if version 9 will open it up.
As Rick says, you are down a one way street. Once you use any new features provided in RH10 then (as with most software) opening the project in an earlier version is likely to cause problems. It's a bit like asking your wife to cook chrismas dinner the same way your ex-wife did it all those years ago - things won't turn out well.
You can indeed remove the .cpd and at least look at the project - that worked in RH9 to RH8.
Would I work with it confidently in a production enviroment?
Would I happily switch between the two version when sharing projects?
More of a project planning (and by the looks of it a project costing) issue than software issue, sorry.
You can reliably downgrade back to RH9 as long as you didn't use any RoboHelp 10 specific functions. (You can even downgrade to RH8.)
See http://www.wvanweelden.eu/articles/downgrade-robohelp-project for an overview for a how to.
Willam , because I didn't use any RoboHelp 10 specific functions, I tried your steps from your website and I was able to open and save my project in RH 9. Thank you!