To answer your question. Rudder shaft base has female ending, which sits over a stainless steel pin and the bottom of the shaft is separated from the mounting by a plastic (vesconite or some such) 'washer.' I don't know whether the rudder shaft can be raised enough for the shaft base to clear the pin,if it can one could then simply slide the old washer up the pin and remove the it and slide a new one in. Fortunately when we slipped Night Cap, we found this area to be ship-shape. If the shaft can't be raised high enough, I imagine one could unbolt the bracket that holds the pin and ' plastic washer'. The upper bush was almost non-existant, but part of it (badly worn towards the forward end) was still in the rudder tube. I manage to break some of it out and partly fit a new bush. Although only partly inserted from above, it is working 100%. I might have another go one day. The top bush is about 1 inch long plastic and is pipe shaped. It sits over the rudder shaft and inside the rudder tube. I guess the proper way to remove it is with the rudder removed and then drive it out with a mandrel or pipe. I think it might have been glued into the rudder tube.
Cheers, John