No you won't. RoboHelp versions are not backwards compatible.
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I just tested and was a bit surprised as I created a project in RoboHelp 9, saved and closed, then opened it up in version 8 without issue.
After examining my settings, I found that I had the option to delete the .CPD upon opening enabled, so likely that had something to do with it.
Helpful and Handy Links
As Rick has indicated, if you delete the CPD you can open a RoboHelp 9 project in an earlier version. However, opening the project does not mean you can continue to work without problems, neither does it mean you cannot continue to work. It will depend on whether you have used anything that is RoboHelp 9 specific.
Be wary of low cost versions. They may be from a reputable supplier seeking to get rid of old boxed versions. They may also be from less reputable suppliers who include a number of "features" not normally part of RoboHelp!
See www.grainge.org for RoboHelp and Authoring tips