You probably have some of this already, but FYI:

Cal Fam Code § 3087 (2009)
Modification or termination of joint custody order

An order for joint custody may be modified or terminated upon the petition of one or both parents or on the court's own motion if it is shown that the best interest of the child requires modification or termination of the order. If either parent opposes the modification or termination order, the court shall state in its decision the reasons for modification or termination of the joint custody order.

"Under 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. 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." Osgood v. Landon, 127 Cal. App. 4th 425 (2005)

Cal Fam Code § 3011 (2009)
Factors considered in determining best interest of child

In making a determination of the best interest of the child in a proceeding described in Section 3021, the court shall, among any other factors it finds relevant, consider all of the following:
(a) The health, safety, and welfare of the child.
(b) Any history of abuse by one parent or any other person seeking custody against any of the following....
(c) The nature and amount of contact with both parents, except as provided in Section 3046.
(d) The habitual or continual illegal use of controlled substances or habitual or continual abuse of alcohol by either parent....

Cal Fam Code § 3040 (2009)
Order of preference in granting custody

(a) Custody should be granted in the following order of preference according to the best interest of the child as provided in Sections 3011 and 3020:
(1) To both parents jointly pursuant to Chapter 4 (commencing with Section 3080) or to either parent....


Michelle - Proud DR Rockette
S: 28JUL07, D'd: 29OCT09
http://tinyurl.com/27j9qo2