Who says we are flying an NG version? Maybe this thread should read: put a 737-700 pilot in a 737-300
(There is no FADEC and a host of other little headaches on the older 737's)
But anyway, you say it's dead easy, but would you really have figured out on your own without causing a little bit of chaos how the autothrust works, how to get manual thrust, what the difference between managed and selected is, when to lower the flaps etc? I don't think so! Too many gotcha's.