As needed because of the withdrawal or addition of a municipality or unincorporated area, population changes, or changes in combinations established under Section 452.573(b), the board of a subregional authority shall be restructured under Section 452.573(a):
(1) each fifth year as of September 1 after the date that the census data or population estimates become available; or
(2) when a municipality or unincorporated area withdraws from or joins the authority.