Mass. Gen. Laws ch. 209C § 20
A court with original jurisdiction pursuant to section three has continuing jurisdiction, upon a complaint filed by a person or agency entitled to file original actions, to modify judgments of support, custody or visitation; provided however, that no modification concerning custody or visitation shall be granted unless the court finds that a substantial change in the circumstances of the parties or the child has occurred and finds modification to be in the child's best interests. Except as restricted by section twenty-three, the court may also modify a judgment to protect a party or child. In furtherance of the public policy that dependent children be maintained as completely as possible from the resources of their parents and upon a complaint filed after a judgment of support, orders of maintenance and for support of minor children shall be modified if there is an inconsistency between the amount of the existing order and the amount that would result from application of the child support guidelines promulgated by the chief justice of the trial court or if there is a need to provide for the health care coverage of the child. A modification to provide for the health care coverage of the child shall be entered whether or not a modification in the amount of child support is necessary. 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 the 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. The order shall be modified accordingly unless the inconsistency between the amount of the existing order and the amount of the order that would result from application of the guidelines is due to the fact that the amount of the existing order resulted from a rebuttal of the guidelines and that there has been no change in the circumstances which resulted in such rebuttal; provided, however, that even if the specific facts that justified departure from the guidelines upon entry of the original order remain in effect, the order shall be modified in accordance with the guidelines unless the court finds that the guidelines amount would be unjust or inappropriate under the circumstances and that the existing order is consistent with the best interests of the child. A modification of child support may enter notwithstanding an agreement of the parents that has independent legal significance. For cases being enforced by the IV-D agency as set forth in chapter 119A, a support order may also be modified in accordance with section 3B of said chapter 119A.
Mass. Gen. Laws ch. 209C, § 20