Any hope for a ticket redo?

No idea
But you most certainly can set up an account with Trenitalia as a US resident.

I have and use both the website and the app.

Jen, I think that you can set up an account on Trenitalia. I know that I have one. Have you applied for the Cartafreccia card for non-residents? This has to be done with a paper application that you complete, scan and email in.
https://www.trenitalia.com/it/cartafreccia/assistenza_ai_soci.html#6
Don’t be put off that it is in Italian. Scroll down the page a bit to the 5th arrow where it refers to non residents in both Italian and English. If you click on this one, the information on the next page is in first Italian and then in English and gives directions as well as a link to the form you need to complete.
Basically, you complete a form and email it in. You do not need a tax ID or Italian address.

When you go to Trenitalia, pick a train, and pick a fare, there is a small i next to the name of the fare that tells you the fare conditions. This will give you a sense on what the costs would be to change your tickets. I looked up Me and you and this is what it said, " The offer cannot be modified after purchase and is non-refundable."

I wanted to mention that sometimes the ticket comes up looking like it is for the wrong day when it is for the correct day. Where are you looking? When I look at my upcoming reservation on the Trenitalia app it says Journey date is 25 September. When I click on the ticket itself, it says 26 September. Could it be that you did put in the correct date and it is this quirk of the app that is throwing you off? When you look at the actual ticket with QR code, what does it say?

In a list of countries, the U.S. comes up under S, I believe, on Trenitalia. "Stati Uniti" is Italian for U.S.

Perhaps, you can talk to a train rep when you arrive to Switzerland and get some help. Often, I have more success when I talk face to face.

I, an American citizen, living in America, was able to set up an account and purchase tickets via Trenitalia without incident. Try again. You can also check out Italo.
Good luck and have a wonderful vacation

Lorie

Jen, I just set up my account on the Trenitalia app after reading comments on your post. You need to scroll down to "Stati Uniti" and then you can register as a US citizen. I don't know why that didn't occur to me when I originally purchased my tickets as an unregistered guest. In my haste, I also purchased tickets for our September trip and the app showed that I purchased seats that were not the four seats with a table in-between. Ugh! So I went on the app to try to change the seats and was unsuccessful. It wasn't until just a few moments ago when I registered, then retrieved my tickets (you can do that by scrolling down and choosing "Retrieve a ticket"), then I used my computer instead of my phone, and voila.... apparently the seats I chose were actually correct, but were not showing correct on the app on my phone. While I can't necessarily help you with your specific question, hopefully if you use the web-version, you might have better success like I did.

Sadly, Jen has not come back to let us know how things are turning out for her. I will say that the post above confirms my experience that the app can be quirky and make things look a little different than they actually are. It pays to double check.

If it turns out the app is a little "quirky", I would think that could be a consideration for getting the "redo". I would certainly ask when you arrive.

THANK YOU all for your help! That ‘United States’ in Italian didn’t pop out at me—I will blame it on the “quirk(il)y” annoying site, the time constraint, and trying to do this while at a convention for high school youth groups with an unbelievable noise level. Sorry I haven’t checked back in until now—after attending said conference, I came home with Covid which laid me up until today. Very thankful that I am OK, and for all the help on the forum. I was able to set up an account, and just ended up buying a second ticket for the correct day. I’ll chalk the extra $ up to a learning lesson & not stress further about it.