Doyle v. Doyle, 258 P.3d 553 (Utah Ct.App. 2011) 2011 UT 42m ¶ 38, “Even an overwhelming case for the best interest of the child could not compensate for a lack of proof of a change in circumstances. See Becker, 694 P.2d at 611 (rejecting bare best interests argument and stating that “in order to reach the best interests standard and reconsider a custody award, there must be a showing that there has been a change in circumstances that is material to the custody issue”).
It occurs to me that “an overwhelming case for the best interest of the child [justifying a modification of the child custody award]” IS proof of a material and substantial change in circumstances–a change in the circumstances of, if not one or both parents, the circumstances of the child himself/herself.
Harper v. Harper, 480 P.3d 1097, footnote 6 (Utah Ct.App. 2021), 2021 UT App 5:
 Unlike an adjudicated custody decree, such as the one in this case, a stipulated custody decree “is not necessarily based on an objective, impartial determination of the best interests of the child, and therefore the res judicata policies underlying the changed-circumstances rule are at a particularly low ebb.” Peeples v. Peeples, 2019 UT App 207, ¶ 15, 456 P.3d 1159 (cleaned up). Thus, “in some cases, a lesser showing of changed circumstances may support modifying a stipulated award than would be required to modify an adjudicated award.” Id. (cleaned up). For the same reasons, in change of custody cases involving a nonlitigated custody decree, the changed-circumstances rule “must not be so inflexible as to categorically foreclose examination of the child’s well-being.” Taylor v. Elison, 2011 UT App 272, ¶ 14, 263 P.3d 448.