Contrary provision as to abatement

1. When provisions of the will, trust or other testamentary instrument of the decedent provide explicitly for an order of abatement contrary to the provisions of section 633.436, the provisions of the will or other testamentary instrument shall determine the order of abatement. 2. Except as provided in subsection 1 of this section, if the provisions of the will, the testamentary plan, or the express or the implied purpose of the devise would be defeated by the order of abatement as provided in section 633.436, then upon application to the court by a fiduciary or a distributee, and after notice to all interested parties, the court shall determine the order for abatement of the shares of distributees in such other manner as may be found necessary to give effect to the intention of the testator. In order to change the order of abatement as provided in section 633.436, it will be necessary for the court to find it clear and convincing that the provisions of the will, the testamentary plan, or the express or implied purpose of the devise would be defeated by the order of abatement stated in section 633.436.