Since it's a DF, you can change the plates through the fire door. You may not be able to get the ring out, but if you can, you can pull the ring and plate assy. out as one piece. Otherwise, you can replace them through the opening. Pulling the entire pot can be a big job and may not go well if the pipe doesn't cooperate and the air chamber won't pull off the air pipe.
If I was going to change the pot bushing, I'd remove the worm and pipe and take off the 4 bolts that hold the bottom of the pot in place.
There is some information about the bushing in this thread:Checking the burner worm bushing
This one is about maintenance:Annual Maintenance and Cleaning of the DF520 Boiler
This one shows replacing the plates through the fire door:Pictorial: Changing the burner plates