In the editor you can right click and Change Type for the parent's Node type if that is easiest.
To move the children, just create the new parent node, select all the children and drag them into the new node. From there if you want that parent to be a root of the scene, you can right click and Save Branch as Scene.
If you're talking about in code, you have to remove the child from the parent first, then add to the new parent.
var parent = self
var child = get_node("child_to_move")
var new_parent = get_parent().get_node("new_parent")