OK, the title may be a bit misleading. But this is how you can try to do it:
For Project RVT files:
- Export from higher version of Revit to IFC
- Import the IFC file into the lower version of Revit
Tip – use the best available IFC format that both versions of Revit understand
For Family RFA files:
- Load the family into a project in the higher version
- Follow the above steps for Project files
- Once you have the project in the lower version of Revit, open the downgraded family from the Project (select in Project Browser – Edit)
- Export to DWG or SAT from the higher version
- Import the file into the lower version
Tip – you probably can’t explode the resulting import, or everything will disappear.
Basic tip via CAD Forum – How to save Revit project to an older version?
Also check out Sharing a Revit File with a Previous Release