generally speaking, mercedes does some sort of facelift/ update/ new engines thing midway through the product cycle and coinciding with the release of a new model. so the introduction of the R129 in about 1990 meant that all the other cars went to a full panel bump strip instead of the black strip that had been used before. also stereos were upgraded from simple four speaker outfits to 6 speaker ones. and so on.
that also meant new engines were available as the 129 used twin cam multi valve engines that only the 190E 2.3- 16 had used prior. however, the twin cam v8s in the 400SL and 500SL were not introduced at that time, since the W140 was still coming. when that was introduced, the 400E was begat. only the 220E, 280E and 320E were employed, and i belive the displacement was bumped in the 220E motor when the 201 was introduced. i dont recall.
basically, mercedes has always updated its cars, not always on a set schedule, and not always with another product launch.
1989 - Full side mouldings with chrome line and chrome line added to bumper tops, wood door trims
Early 1993 - 220E, 280E and 320E introduced to replace 230E, 260E and 300E respectively (although the 220E was really designed to replace the 260E and the 280E the 300E)
Late 1993 - New bonnet with smaller grille and headlight surrounds, re-shaped headlight glass, new tailgate, new bumpers with longer rear bump strip, clear front indicators and smoked rears. New colours, all bumpers fully painted.
1989 also saw body colored side mirrors; newly designed bucket seats with softer leather, and the windows and sunroof could be closed using the key (hehe, not really a face lift, just interesting)
1993 carrying on to 94 saw rebadging (300E - E320, 500E - E500)
I have information for what was changed/ modified/ added each year if you have any questions involving change that pertain to a specific year.