Yes, you can save QuarkXPress document one version back. Such as, QuarkXPress 9 can save in 9 or 8 format. QuarkXPress 8 can save in 8 and 7 format.
Most versions of QuarkXPress can open QuarkXPress files back from version 3. You cannot open files from a newer version in an older version. For example, QuarkXPress 8 files cannot be opened in version 7 but can be opened in version 8 or higher.