It is not necessary to remove the hub from the spindle to replace front rotors on the 124 series car. Simply remove the wheel, unbolt the caliper and suspend it out of the way with a wire, don't hang it by the hose or let any tension be placed on the hose. Then remove the allen head screw that holds the rotor to the hub, and remove the rotor.

Before placing the new rotor on the hub, clean the hub THOROUGHLY with a wire brush and get it shiny, squeaky clean. Put the rotor in place on the hub and put the allen screw back in place.

All that said, this would be a perfect time to go ahead and remove the hub, clean everything up in solvent, repack the bearings and put it all back together. You can get a tube of grease from the MB dealer for not too much money that will have just the right amount for both front hubs, so you will know that you put in the correct type and amount of grease.

If you have any further questions, post them here.

Hope this helps,