There's WAY more difference in the 2388 production run than there is between the last 21 and the first 23.  The really valuable upgrades in my experience with those machines really started about MY2001.  At that point, you have the hydraulic feeder reverser, a larger fuel tank that will actually let you run all day, external sieve adjustment, 2-speed spreader.  AFX rotor came next.  If you get into the last 23s then you have extended wear auger package factory-installed.  Also, the paint on the Grand Island machines is far superior.  All of this and more are listed on the Hoober combine timeline, but those are the highlights of improvements that made a difference out in the field through the several different 23 series combines we ran.