Where is the journey on the 20th?

I think you’ll have to buy a ticket for travel that day. She won’t be eligible for the Senior pass until the 21st.

If you post the journey you’re making, and the time, we can have a look for the best solution. It’s unlikely the seat reservation you’ve already bought will be of any use I’m afraid.

What journeys are you making before the senior pass will become useable on the 21st?

Another option would be to exchange the Senior pass for an Adult pass (will cost €15 exchange fee, plus the difference in price) then she can use the reservation.

Obviously the adult pass will cost more, so it might be cheaper just to buy a new ticket on the 20th.

https://www.eurail.com/en/eurail-passes/ordering-info/refund-exchange-policy

Looking through your previous posts, I guess you’re doing a day trip Paris to Strasbourg on the 20th? 0716 departure, 2026 return.

You can get a ticket on the outbound train - €130 for first class, but unfortunately the 2026 return is completely full. There is a slightly earlier departure on Ouigo (low-cost operator) at 1948. The Eurail passholders can’t travel on that, but the Senior could travel alone (€58, second class only).

The pass exchange will probably be the cheapest option, even if you bought the pass during the Black Friday sale and now have to pay full price.

Regular tickets would cost at least 115€ + 55€ and the person won't travel with you. Meanwhile exchanging passes might cost 150€ but you get to stay together (reservations remain valid as they're tied to a name, not a pass number).