This question has been asked numerous times on the forums, so I'm posting it to the FAQs where it should be readily found:
Revit cannot SAVEAS or save down to previous versions. There is no way to open a 9.0 file, and SAVEAS to a Revit 8.1 file. There has never been this abiltiy in Revit, and probably never will be, due to the fact that Revit is sold almost entirly on subscription. Upgardes are included as part of the upgrade, and each new version is feature packed and well worth upgrading to when it's released.