What is the latitudinal and longitudinal exetnt of Savannah grassland?

What is the meaning of Except as respects things done or omitted to be done before such supersession?

My answer is: Let us break the statement in small parts. Logical conclusion would be as under: 1. An existing order is there 2. This existing order is superseeded or replaced by new order. 3. But how much? To such an exetnt that where the replacing or new order is silent, the prevalent rules and process are to be followed. i.e. what the phrase "Except as respects things done or omitted to be done before such supersession" suggests. Hope you will agree.