Seems like two different questions. How are a form/subform link? In Design view, if you select the subform (squares around the perimeter) and look at the property sheet, you'll see Link Child Fields and Link Master Fields. You'll see the connecting fields.
How do you change the parent of a child? Just change the foreign key in the child record. That should also be the primary key in the parent. In fact, look at your terminology - parent/child, just have another parent adopt the child.