I did my 737 training only on classics (300/400) never even sat in an 800 and my licence has 737-300 to 800.
So the way I understand it is realistically I would have to do a differences course to fly an 800 for some one but if I had the $$$ I could go and buy my own 800 and fly it tomorrow , like I said my licences has 800 on it
Cheers noted