It depends on the refund policy of your bought ticket. Surely non-refundable for advance tickets.

You can still buy a pass and use it for later journeys

What would be the route, date and departure times ? Usually there are seat reservations available as long as you can buy regular tickets for that train. Exception : Thalys, Eurostar and TGVs to Belgium (quota). Seat reservations can (and should) be bought somewhere else than Eurail for better prices (usually with the railway companies themselves). For that you do not have to have bought a pass beforehand

You can check availability for them here though : https://www.b-europe.com/EN/Booking/Pass#TravelWish

General advice : https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105

Let me know for questions

Thank you for your reply. I checked the link but can’t seem to locate how to check the overnight train options.

We currently have Eurostar tickets from London to Amsterdam (3 people) does not state refund just info about changing ticket for the difference.

We still need: 

Amsterdam to Zurich (overnight train) 6/2 depart

Zurich to Lucerne 6/3 morning

Lucerne/Zurich to Paris 6/6

Paris to London 6/9

Sleepers/couchettes to Zurich are fully booked. Seats still available.

Couchettes to Basel are still available, no sleepers anymore. In Basel, change to a seating carriage that continues to Zurich.

Book on tickets.oebb.at. Add the Interrail/Eurail discount for every person and then select "One-way tickets”.

No reservation needed. Look up timetables on sbb.ch.

Pass reservations from Zurich/Basel ro Paris are extortionate.

Therefore, seats from Mulhouse or Strasbourg to Paris here:

https://travel.b-europe.com/Eurail-GE/en/booking-tgv#TravelWish

Take reservation-free trains from Switzerland to Mulhouse/Strasbourg.

Book here:

https://www.b-europe.com/EN/Booking/Pass#TravelWish

Availability is still quite good, but since it's a Friday, book soon. Eurostar pass holder seats can be fully booked weeks in advance.