A pretty close rule of thumb for the OP's leftover ticket is it will currently be worth 2/4's (one half) of the original purchase price of the 4 day PH. Don't ask to upgrade it though or they might say you can't. You want to apply the remaining dollar value of the old ticket towards the AP.
You only had 7 days from first use to upgrade it. That's when they apply the full purchase price towards the AP but backdate it to the first day you used the PH.