Here is some information from

http://en.wikipedia.org/wiki/Mercedes-Benz_W126

Four years after the introduction of the fuel-efficiency "Energy Program," the model range had been reworked completely. In September 1985, again at the IAA in Frankfurt, the reworked model range was reintroduced. Apart from visual changes to the bumpers, side covers and alloys, the changes made to the available collection of engine variants was most visible. Two newly-constructed 6 cylinder engines and new 4.2 and 5.6 litre V8's were added, and other engines were further upgraded.

So I think the mechanicals changed but the bodies didn't alter too much. Although if you scroll down from this quote on Wikipedia you'll see a whole list of different chassis numbers...