I have tried something similar before, but it was not to a 3.9.0 installation. It worked pretty good for the content as far as I could see and test, but related objects were messed up. The related objects were mainly images, so articles showed the wrong images or no images at all. I think because eZ Publish assigned new/different object id's to them at import.
My advise is to test it on a local 3.9.0 installation first.