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You're looking here at the Fare Difference and any change fee on your flights as booked, I would guess there will be cheaper options if you are prepared to look at other flights on the same day. This is different from POUG and AUP pricing (for which you'll get indicative prices in their respective threads in the Dashboard), and they show up when they really want to make a bit of money out of you. (And I agree that First isn't going to be the right fare there). At the moment I don't think they want you to move, so pricing it accordingly.
Note that even with POUGs/AUPs, these are priced above the cheapest CW (or whatever) fare available on the route, and deliberately so, if you intend to travel in a better cabin the logic is to encourage you to book this in the first instance.
And the late Thursday flights have a few weekly/fortnightly commuters. Add in a American expat families returning home for Christmas then there may be a bit of a CW price spike in evidence in that more seats are taken than expected (not knowing the actual date, so I haven't checked).