will the work already done on the image quality have to be done again?
When the D800 (and D4) become available they will get additional testing.
The chief reason is that the "gold standard" for my measurements requires taking photos of a specific test target or is a lens/body cap shot. We don't have these for the D800 or D4. Often with a proper test target results are better, it's seldom that they are worse.
Because I'm using NEF files for my measurements it's highly unlikely that any firmware changes will have an effect. I think there's a low probability that last minute firmware changes would affect JPG output but not the NEFs.