1) the change would be "in the best interest of the child", AND
2) the circumstance of the child, you or your ex have "materially and substantially changed since the earlier of:
(a) the date the order was signed (rendered);
(b) the date the mediated agreement was signed.
Another circumstance which would allow a modification is if your child is at least 12 years old and has filed in court, in writing the name of the person they want to live primarily with.
Yet another circumstance is if the person who in the old order had the exclusive right to designate the primary residence of the child, gives up that right for at least 6 months.