65 ILCS 5/5-5-5
Any city or village which has adopted this Article 5 and was operating under Article 4 at the time of such adoption may upon abandonment of this Article 5 also abandon operation under Article 4, as provided in Section 4-10-1, and by so doing shall become subject to the alderperson form provisions of Article 3 and shall be subject to the provisions of that Article 3 the same as if it had been operating under Article 3 at the time this Article 5 was adopted, except for any period of time after abandonment of this Article 5 necessary to make the provisions of Article 3 fully and completely applicable.
Any city or village which has adopted this Article 5 and was operating under Article 3 at the time of such adoption may upon abandonment of this Article 5 also abandon operation under Article 3 by adopting Article 4, as provided in Sections 4-2-2 through 4-2-9, and by so doing shall become subject to the provisions of Article 4 and shall be subject to the provisions of that Article 4 the same as if it had been operating under Article 4 at the time this Article 5 was adopted, except for any period of time after abandonment of this Article 5 necessary to make the provisions of Article 4 fully and completely applicable.
65 ILCS 5/5-5-5