Sorry Lieve, but it's the other way round. By definition Captivate is backward compatible because later versions of Cp can open earlier version files (as long as they are not several versions old).
Until Cp7 it just wasn't forward compatible.
http://www.infosemantics.com.au/adobe-captivate-troubleshooting/backwa rd-forward-compatibility
And the jury is still out as to whether Cp6-Cp7 is completely forward compatible.