Trainline Tickets Exchange?

It seems you can change your ticket, but there may be a fee and the train you want to change to may be more expensive - you will have to pay the difference.

Trainline is easy to use and a good option when folks have difficulty with the country specific train company. But, it is a third party. You'd have better luck exchanging that ticket had you purchased from SNCF or Oui. Oui will likely say you need to work with Trainline. I do have experience with this. I cancelled a trip during COVID and France actually closed borders so my train travel was essentially cancelled. I had paid about $80-$100 for high speed train tickets. I tried to get a refund. SNCF told me I had to work thru Trainline. Trainline refused to refund or give credit and told me to work with SNCF. I finally gave up.

Maybe since you will be there at that counter you'd be more likely to be able to get an exchange, I wouldn't count on it, but its worth a try

Being able to refund or exchange a ticket is a function of the type of ticket you purchased. I would ask about your options at the ticket counter at Gare de l'Est or at one of the many SNCF boutiques.

If you had bought directly from SNCF, you would have been able to change your ticket on the fly using the application on your phone by paying a fee and difference in price, and if space were available for the earlier train. But you used a 3rd party seller so you need to contact Trainline to see what their rules are. If they say no, I'd still go into the SNCF office. The worst is they also say no and you wait for your 4 pm train.

The ticket you got from Trainline should be an official SNCF ticket, so you can try adding it to the app using the reference number and the last name.

Then you can see if you can change the ticket...

Where are you arriving from? If by air there are also trains from CDG to Strasbourg. If by train you should try to get a through ticket from wherever you are coming from to Strasbourg.