In California, the non-custodial parent has the burden of proving that the move will be harmful to the child. If the move is determined harmful, then the non-custodial parent could be awarded custody. However, in cases where the parents have joint custody, the court must determine what is in the best interests of the child to come up with a new custody arrangement. Since the parents are so far away, frequent exchanges are not likely to be in the best interest of the child, so one parent is likely to have the majority of parenting time.
The court will likely look to many factors to determine what is in the child's best interest. The first factor the court will consider is which outcome provides the most stability and continuity for the child. If the child has spent the majority of their time with the moving parent since birth and is most comfortable with the moving parent, that could be a factor in favor of the moving parent. Conversely, if the child has laid many roots in the current area, has friends and family locally, and would benefit from staying in that environment, that could be a factor in favor of the non-moving party. The court could also examine the child's relationships with both parents, the parents' ability to co-parent, the age of the child, the distance of the move, the reasons for the move, and where the child wants to live.
Ultimately, a move-away case is a very complicated case, as both parents could be great parents and could be amazing at co-parenting, but frequent exchanges just simply wouldn't be possible. Unlike typical custody cases, the court cannot create an equal timeshare, as such frequent exchanges at a long distance would not likely be in the child's best interest.