“By statute, the district court has continuing jurisdiction over the subject matter of a divorce and may later make such changes in custody provisions as it determines are “reasonable and necessary” for the welfare and “best interests” of the child. U.C.A., 1953, ss 30-3-5(1), 30-3-10. However, since a custody decree is predicated on a particular set of facts, that decree is res judicata and will not be modified in the absence of a showing of a “substantial” or “material” change of circumstances which warrants doing so. Trego v. Trego, Utah, 565 P.2d 74, 75 (1977); Smith v. Smith, Utah, 564 P.2d 307, 309 (1977). Thus, this Court has held that where there is no allegation of a substantial or material change of circumstances, a petition to modify a custody decree should be dismissed. Perkins v. Perkins, Utah, 522 P.2d 708 (1974) (visitation rights). See also Cody v. Cody, 47 Utah 456, 469, 154 P. 952, 957 (1916) (custody, child support, alimony, etc.). Similarly, we have said that a custody decree should not be modified unless changed circumstances are demonstrated. Nielsen v. Nielsen, Utah, 620 P.2d 511, 512 (1980); Anderson v. Anderson, 13 Utah 2d 36, 39, 368 P.2d 264, 265-66 (1962).”