I don't think it is worth installing older versions of Paratext. You won't be having much extra data to preserve, unless Paratext 7 preserved your key terms choices, but even then, the key terms lists have changed a lot since then.
Just create a new project in Paratext 9 and import the Paratext 6 files as text. You may need to verify that each file in Paratext 6 contains the correct \id ABC line at the top. You will also need help importing if the books are set to be one file per chapter. Otherwise, the import should be straight forward, and you will have to fix the markers it doesn't recognize with search and replace.