IN the Swiss only some minor local/regional trains have 2nd only-you mention none of these.

If you can upgrade pass/2nd to 1st for 1 trip depends on country etc-often this is not allowed.

The pass is NOT valid at all up that extremely steep expensive trip in mostly tunnel up to JF-but gives areduction only-as do many other ways too. You also need extra-expensive RES if you want to use TGV over the border-can reduce that by using only local trains for last hop there.

TGV Lyria Paris-Geneva has very expensive reservations in 1st class, also when you have a 1st class pass.

As a recent user of a first class pass on a 3000 km trek from Sicily to the UK we really appreciated the First Class experience. Space and peace with varying levels of catering. Plenty of luggage storage etc. 

However these were long distance trains from 2 hours up to 10 hours.

On predominantly short journeys (up to 2 hours) I don’t feel it makes a lot of difference.

Only you can decide on value for money, but I feel the premium on both the pass and some reservations is worth it for our needs. (A mature couple with decent savings).

If the costs of the premium are an issue then obviously stay in 2nd class for exactly the same Trip.

Hmmm, for that leg I wouldn’t consider it.

TGV has a more comfortable and wider seats in first, but no extra at-seat service or newspapers like you have on some other trains like Railjet or ICE with 1st class “or standard première” in central Europe. Business première is not included in the pass. The longer the journeys, the more worthwile 1st class gets. Your trip isn’t too long.

And indeed, very expensive reservations, like Angelo said. Not worth the price difference in my opinion. Second class TGV has also comfortable seats.

Local trains in France and Switzerland often don’t have a first class. If you really fancy a bit of quietness and comfort you can consider it, but if you travel off-peak and off-high season it’ll be equally quiet and laidback in second class.

Note that the Jungfraubahn and the BOB are not included in your pass. You get a 25% discount though. And Jungfraubahn rates are astronomically high (and don’t even have 1st class), but quite the experience I gather. Never tried it though.

If you plan well in advance, you might be cheaper off buying separate tickets than a pass for the trip you’re planning a bit in advance.

Especially with the TGV you can do some deals every now and then, perhaps even in 1st.

Just a little remark: In Switzerland ALMOST all regional trains have first class… But as you are usually just for a short time on them, doesn’t make a huge difference.

Jungfraubahn itself is not so interesting… big tourist rip off. You are from Eigergletscher on all the time in a tunnel and see exactly nothing. Just up there you have a viewing platform and can walk a bit around in the snow. There are similar experiences for a much lower price in other places/areas.
Although the way up from Lauterbrunnen/Wengen  to Kleine Scheidegg/Eigergletscher is really nice. But that you can also do as a nice hike.