It depends what year your S420 is, from 92-95 the whole shaft and shift knob are one peice, if you have a 96-99 there is a little black peice that swivels at the base of the shifter that lets you remove the shifter itself of the chrome shaft. You also have to keep this in mind for what type of shift knob you want to get.
I personally changed the shifter of my 95 S420. It was tricky sice I wanted to change it to the 96+ style S600 (knob dosn't come with shaft). I cut the original knobs stiching off and carefully removed the knob from the shaft.Then I simply slid the new one on the shaft and tightning the black swivel base.
Hope this helps.