Hmmm - let me try a different explanation then:

Approval and section membership are two different things in the DB (unfortunately).

Although the two, approval and section membership are different, they still need to be in concord (again, unfortunately).

A node of type a needs a matching approval into section A. If any approval already exists, no other approval can be added.

So, if a node of type a has approval A but then gets changed into type b, it cannot be approved into section B until the approval from section A is removed.