I had a 2007 and a 2008 RDX. I am looking at used 2009 - 2012 RDX and notice the DOT mileage ratings are better in the 2010-2012 models vs 2007-2009 models. What was changed in the 2010 up models that made the gas mileage ratings better than the 2007-2009 models?

I don't think so....if they do is because the changed the test in 2008, and actually the ratings went a bit down from previous year model cars (for all brands/models) simply because of the change.
The RDX is the same car from 2007 to 2012, mechanically speaking, in 2010 they introduced the FWD or 2WD model and that one has higher ratings, maybe you are looking at that.

Thanks, we didn't have the fwd only model up here so it must be a testing change.