Mo. Rev. Stat. § 452.1218: Modification of agreement.

1. By mutual consent, the parents of a child may modify an agreement regarding custodial responsibility made pursuant to sections 452.1214 to 452.1222.

2. If an agreement is modified under subsection 1 of this section before deployment of a deploying parent, the modification must be in writing and signed by both parents and any nonparent who will exercise custodial responsibility under the modified agreement.

3. If an agreement is modified under subsection 1 of this section during deployment of a deploying parent, the modification must be agreed to in a record by both parents and any nonparent who will exercise custodial responsibility under the modified agreement.