Section 31-17-2-21 - Modification of child custody order(a) The court may not modify a child custody order unless: (1) the modification is in the best interests of the child; and(2) there is a substantial change in one (1) or more of the factors that the court may consider under section 8 and, if applicable, section 8.5 of this chapter.(b) In making its determination, the court shall consider the factors listed under section 8 of this chapter.(c) The court shall not hear evidence on a matter occurring before the last custody proceeding between the parties unless the matter relates to a change in the factors relating to the best interests of the child as described by section 8 and, if applicable, section 8.5 of this chapter.