If you have already paid extra for a seat, and you decide to pay again for a more expensive seat, the first payment is subtracted from your new seat fee.

So, if you paid the fee for a seat assignment and decide to pay again for an exit row, you are credited for your first payment. Same thing if you upgrade from either of those to business class.

If you forfeit any paid seat to take a seat without a fee, you also forfeit the original fee.

I have not seen a written policy...this is just my anecdotal experience.