You should be able to open it in RoboHelp 8 after locating and deleting a file with the project name and ending in .CPD. Please note that any feature specific to RoboHelp 9 will not work.
Helpful and Handy Links
Van: Captiv8r email@example.com
Verzonden: dinsdag 13 december 2011 15:27
Re: from RH8 to RH9 but now to RH8 again