(1)    By mutual consent, the parents of a child may modify an agreement regarding custodial responsibility made under this subchapter subject to any visitation rights granted by court order under s. 767.43.

   (2)   If an agreement is modified under sub. (1) 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 sub. (1) 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.