this might sound cheeky but i cant think of any other way to put it, take the old ones off, put the new ones on.
Unbolt back box clamp and remove, unhook of hangers, thats that out the way
unbolt clamp on centre section and remove, un hook off hanger, done.
Put new centre section on rubber hanger, clamp up. Put new back box on hanger and clamp. done.