It appears that Eurostar has relaxed their rebooking deadline for future travel. Previously you had to rebook NLT 31 March 2021. Here's the updated policy from their website for info:
"To provide you with flexibility and peace of mind we are able to offer you an eVoucher if you are due to commence your outbound Eurostar journey between Friday 13 March and Wednesday 1 July 2020.
We will offer you an eVoucher equivalent to the full value of your current Eurostar train or Eurostar package (train + hotel) booking. Your eVoucher will be valid for 12 months from date of issue to travel to any Eurostar destination (subject to availability).Please note that you do not have to travel before the expiry date on your voucher, you simply need to book your new journey before your voucher expires. You can make a booking up to six months in advance on most of our routes meaning you can travel up until late 2021.
No fees are applied but if the value of your new booking is greater than the value of your eVoucher, we’ll ask you to pay the difference.