OK, the title may be a bit misleading.  But this is how you can try to do it:

For Project RVT files:

  1. Export from higher version of Revit to IFC
  2. 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:

  1. Load the family into a project in the higher version
  2. Follow the above steps for Project files
  3. Once you have the project in the lower version of Revit, open the downgraded family from the Project (select in Project Browser – Edit)

or //

  1. Export to DWG or SAT from the higher version
  2. 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

This is one of the most exciting blog posts I have read for some time:
http://revit.blogspot.com/2010/03/accessing-project-parameters-in.html from Jay Polding at revit in plain english.

Essentially, in any schedule you can actually add Project Information parameters. This includes Sheet Schedules, Component Schedules and Material Takeoffs, but not Note Blocks. All you need to do is tick the ‘Include Elements in Linked Files’ box to expose the ‘Project Information’ option in the dropdown list. And when you add custom Project Information parameters, make sure you tick ‘Instance’ in the Parameter Properties box (otherwise you won’t see Project Information as a possible category for your new parameter).

My brain is starting to tick over as to possible uses of this in filtering and calculation… You could use it to set a provisional unit cost for flooring, use the parameter in a number of different cost schedules, and then 6 months down the track if the cost changes – you only have to change it once, in Project Information!

You could filter all of your schedules with a Yes/No Project Information parameter, lets say for ‘optional building elements’, and when the Client decides they don’t want them – just untick the box in Project Information and all the schedules hide all of these optional elements. That is cool!

Sometimes you really want to copy some model lines from a project RVT to a family RFA, but Revit says NO!

This is the workaround:

  1. In the Project, create detail lines that describe the elements you want to copy
  2. Make New Family – Generic Profile
  3. Copy the detail lines in the Project (Ctrl + C)
  4. Paste them into the Profile family (Ctrl + V)
  5. Open / create the other family you want to put those lines into
  6. Copy them from the Profile family into the other family (lets say it is a Planting family). They will appear as model lines in the Planting family. Yay!

Sure, this method has some limitations, but it is a valid workaround in a lot of situations.