Is this possible?
Solved
Can I change my passport details after booking seat reservations for eurostar?
+6Through Eurostar's own website, eurostar.com, where you can log in with your booking reference and name.
Can it be updated if the seat reservation is done in Eurostar when the seat reservation is done through Eurail global pass? I am get a new passport, but planning to reserve a seat before I get the passport 3 weeks later
Pass reservations can't be done on the Eurostar website.
However, regardless of where you booked your Eurostar reservation, you can add or change your passport data on the Eurostar website.
Enter your E-mail address. We'll send you an e-mail with instructions to reset your password.