What is the changed circumstance test and what is the effect of the changed-circumstance rule?

"[O]nce a final judicial custody determination is in place[,] [u]nder the so-called changed circumstance rule, a party seeking to modify a permanent custody order can do so only if he or she demonstrates a significant change of circumstances justifying a modification." (Montenegro, supra, 26 Cal.4th at p. 256.) "The changed-circumstance rule is not a different test, devised to supplant the statutory test, but an adjunct to the best-interest test. It provides, in essence, that once it has been established that a particular custodial arrangement is in the best interests of the child, the court need not reexamine that question. Instead, it should preserve the established mode of custody unless some significant change in circumstances indicates that a different arrangement would be in the child's best interest[s]. The rule thus fosters the dual goals of judicial economy and protecting stable custody arrangements." (Burchard v. Garay (1986) 42 Cal.3d 531, 535.)