My 10 year old MK Maxxum has bit the dust so I want to upgrade to a 36v. What new is required wiring wise. I understand in general another jumper has to be added from Batt 2 to new Batt 3. Any need to upgrade wiring? Change the trolling motor plug receptacle?

In general yes. Just add the jumper for the third battery. If your wire is the appropriate size for the 24v motor you should be good for the 36v motor. A 36v motor will draw less current than a 24v motor of the same thrust rating.