Where did you book your reservation?

I booked it from the Eurail website

Unless rian knows a better workable solution, you probably best accept this is money lost and buy simply a new one for 3/8. Also note that it is ONLY for direct EC trains needed-cost 11€, and its very much possible to use local train till border It-Swiss (Mostly Chiasso) and then board onward train-for free. This will take thus some more time-you have to leave earlier-and its not really feasible if you also use IT-fast trains to reach Milano-then Swiss

If youre overworried again about ´full trains´-and depending how long advacne you are in IT-just go to counter and ask if they can arrange it-be prepared that they say no va.

I'm not sure what refund/exchange conditions Eurail use but normally these are set by the operators so I'd expect them to be similar. That would mean an exchange or refund is possible before departure, with 20% costs. The easiest may be to initiate a refund from the Reservations Overview and then book a new one via ÖBB (no booking fees): do NOT use "Seat only” but add the "Interrail / Eurail” discount and then look for normal tickets. Price is 11€.