Section 13. (a) Any payment or installment of support under any child support order issued by any court of this commonwealth or by a court or agency of competent jurisdiction of any other state shall be on or after the date it is due, a judgment by operation of law, with the full force, effect, and attributes of a judgment of this commonwealth including the ability to be enforced; shall be entitled as a judgment to full faith and credit; and a judicial or administrative action to enforce said judgment may be commenced at any time; provided that said judgment shall not be subject to retroactive modification except with respect to any period during which there is pending a complaint for modification, but only from the date that notice of such complaint has been given, either directly or through the appropriate agent, to the obligee or, where the obligee is the plaintiff, to the obligor.

(b) After a judgment pursuant to chapter two hundred and eight, two hundred and nine or two hundred and nine C, an agreement between parents that provides for the support of a minor child shall not be enforceable to bar a modification of the amount of support due on behalf of the minor child unless the court finds all of the following: 1) that the agreement survives the original judgment and has independent legal significance; 2) that the agreement was fair and reasonable and free from fraud and coercion at the time of the judgment; 3) that the provisions for support of the minor child continue to be fair and reasonable considering the child support guidelines and the circumstances of the parties and the child; and 4) that enforcement of the agreement is in the best interests of the child.

(c) In any proceeding to establish or modify an amount of child support, the child support guidelines promulgated by the chief justice of the trial court shall apply. There shall be a rebuttable presumption that the amount of the order which would result from the application of the guidelines is the appropriate amount of child support to be ordered. If, after taking into consideration the best interests of the child, the court determines that a party has overcome such presumption, the court shall make specific written findings indicating the amount of the order that would result from application of the guidelines, that the guidelines amount would be unjust or inappropriate under the circumstances; the specific facts of the case which justify departure from the guidelines and that such departure is consistent with the best interests of the child.

(d) The provisions of this section shall apply to all actions for establishment, modification or enforcement of a judgment or order for child support pursuant to or adjudicated under the provisions of chapters one hundred and nineteen, 119A, two hundred and seven, two hundred and eight, two hundred and nine, two hundred and nine A, two hundred and nine C and two hundred and nine D.