YUP fares ARE upgrades
They simply allow you to upgrade at the time of booking so it appears that you booked into First. However, your ticket is still Coach.
On your travel date, if you end up getting re-routed due to delays or cancelleations, your new flights will most likely be in coach. If transferred to a different airline, you will be in coach. Period.
You will not be entitled to a partial refund for riding in coach, even if a lower "non-YUP" fare was available when you decided to pay higher for the YUP.
And of course the fare basis code of YUP translates to "Coach Upgradable".