(1) Process against an incompetent shall be served:
(a) By serving two copies of the process to the person who has care or custody of the incompetent or, when there is a legal guardian appointed for the incompetent, by serving the guardian as provided in s. 48.031.
(b) By serving the guardian ad litem or other person, if one is appointed by the court to represent the incompetent. Service on the guardian ad litem is unnecessary when he or she appears voluntarily or when the court orders the appearance without service of process on him or her.
(2) In all cases heretofore adjudicated in which process was served on an incompetent as prescribed by any law heretofore existing, the service was lawfully made, and no proceeding shall be declared irregular or illegal if a guardian ad litem appeared for the incompetent.