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