Section 455.065 - Subsequent modification of orders1. Provisions of any order respecting maintenance or support may be modified only as to installments occurring subsequent to the motion for modification and only upon a showing of changed circumstances so substantial and continuing as to make the terms unreasonable.2. Provisions of any order respecting custody may be modified only if the court finds, upon the basis of facts that have arisen since the prior order or that were unknown to the court at the time of the prior order, that a change has occurred in the circumstances of the child or his custodian and that the modification is necessary to serve the best interests of the child.3. Provisions of any order respecting visitation may be modified when the modification would serve the best interests of the child.