(a) The council of the larger city or incorporated town shall cause a plat to be made of the entire city or incorporated town after the annexation thereto and the division into wards of the smaller municipal corporation.
(b)
(1) A certified copy of the plat shall be filed and recorded in the office of the circuit court and ex officio recorder of the county and with the Secretary of State.
(2)
(A) Thereafter, the plat shall stand, be, and remain the division of the city or incorporated town into wards, and the number and boundaries thereof, until such time as it may be afterwards changed according to law.
(B) However, a change in the boundaries of the wards of the larger city or incorporated town shall not determine or affect the time of service of any previously elected council member of any ward in the larger city or incorporated town.