The commentary also addresses conversion of modal propositions. Necessity, permanence, actuality and possibility change a proposition’s truth conditions. Therefore a modal qualification cannot automatically be carried over unchanged when the terms are exchanged.
Being true at all times is a stronger claim than being true at some time. A valid converse of a modal original may have a weaker modality.
The source gives conversions of several modal forms under general absoluteness (muṭlaqa ʿāmma) and notes exceptions. Understanding each modality’s scope is essential before attempting detailed modal conversions.