Until you activate your pass and transfer your train from the pass you simply add and delete trains as you wish. If you have added a train to your pass from your trip and then decide not to travel for any reason you simply deactivate that train in your trip and add the replacement.

There is no reason to activate your pass and transfer trains until shortly before boarding the first train. 
So for example, if you added a train at 1030 to your trip and actually arrived in time to board a train at 1000 you simply delete the 1030 and add the 1000.

Obviously trains requiring reservations need the reservation changing in the ticket office before boarding, but that is totally separate from your pass.
 

In Germany and Switzerland you don’t need reservations. Put all the trains you might take into your Trip, and then just select the one you end up taking. 

you can do it just before you board

These are the first three reasonable connections for tomorrow, for example. I’d add all three to my Trip, and then quickly slide the switch to yellow when I knew which train I was taking, to show which one I was taking. If you want to add a few more later options, do so  

You don’t need any data to do this, once you’re on the ground, but it helps to have your phone on airplane mode. 

You can continue to edit your journeys - even after you’ve boarded. So you could board the first train and then make use of the WiFi or you data plan to sort out the rest of your journey

Thank you Ralderton and Yorkie! That helps. Regarding reservations in Germany, isn’t there a good chance that 1st class from Frankfurt to Basel would be sold out May 29th? Os is that later in the season? 

If you search on www.bahn.com/en it will give you an indication in the results of how busy they expect each train to be - at least for Germany.

You could make a reservation on a later train if you like, just so you have something ‘in the bank’. You don’t have to use it. You can still jump on an earlier train.

But I wouldn’t bother. In my experience, there’s always space. If nothing else, go and sit in the dining car and order a second breakfast!

I love the diner car! And I had to do that last summer in Germany at the height of the 10-Euro mania. But that was summer. 

As for Switzerland, a few have told me that you don’t need reservations, but when I look at a journey from Bern to Venice for May 22nd, it says “seat reservations required” (on the app). Furthermore, when I go to the SBB website, I see no option to just by seat reservations (I had no trouble with this on OBB or DB). Maybe I’m missing something???

Venice isn't in Switzerland :)

So you'll need a reservation for the train from Switzerland to Italy and any Frecce or IC in Italy. SBB don't sell reservations for those online but ÖBB do on tickets.oebb.at. Do NOT use "Seat reservation only” but add the Interrail/Eurail discount and then choose "One-way tickets and day tickets”.

Within German and Switzerland reservations aren’t compulsory.

On cross-border trains from Switzerland-Italy, they are. And on domestic trains in Italy they are compulsory for the fast (Freccia) trains and Intercities.

Ha! Yes, so did I. At Munich, trying to get on the train to Innsbruck (probably not €10 travellers, since it was an EC, but maybe half-price Interrailers). A swarm of people on the platform crowding the doors.

Luckily, I was travelling light, so I could make a beeline for the dining car and had the carriage almost to myself - while everybody else sat on their luggage.

Thanks, Ralderton! 

Yeah, Hamburg last summer was a madhouse; we took a regional train that was filled to the max with crowds staning. It got rowdy; there was a screaming match, the train stopped, the police came. Adventure. :) 

Check bahn.com like others say to know how busy the train could be. Keep in mind that in 1st class people that book “normal” tickets gets the reservation for free. So could be full of reservations, but because German railways have every time delay people can’t reach the connecting train or other problems. A seat reservation in Germany is valid only 15 minute after departure. If no one sits on it after this time you are allowed to use it.