The merger shall be effective thirty days after the effective date of the ordinance annexing the territory within the sanitary and improvement district. If the validity of the ordinance annexing the territory is challenged by a proceeding in a court of competent jurisdiction, the effective date of the merger shall be thirty days after the final determination of the validity of the ordinance. The trustees or administrator of the sanitary and improvement district shall continue in possession and conduct the affairs of the district until the effective date of the merger, but shall not during such period levy any special assessments after the effective date of annexation.