What is the changed-circumstance rule?

"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." (Burchard v. Garay (1986) 42 Cal.3d 531, 535.)