Many confused people not reading OP's post properly "help" in making this extra confusing... 😮
Anyway, you cannot compare procedures before and after final pay. This cruise is past final pay, so the "pay the difference" using a "price drop-price" as your guide should be out the window just by that fact. No automatic "cancel and rebook" without a penalty at that point! (Even though I'm sure it has been done many times. But still not required)
The $800 quuote probably has a "cancel after final pay"-fee (or something of that nature) built in.
Everyone, incuding OP, please drop the "slider thing". It has absolutely nothing to do with this! It could be that "slider maximum" IS the upgrade price, but no point in getting stuck on that!