This information relates to the 4 trains which are cancelled tomorrow but the same principles will apply on other dates:
"You are invited to exchange your ticket within 60 days, regardless of ticket type, for another service within our booking horizon. If you are holding a flexible ticket, please do so at www.eurostar.com, otherwise please call the Eurostar
Contact Centre on 08432 186 186, or if calling from outside the UK, +44 (0) 1233 617 575. Alternatively, please present yourself to a member of staff at your departure station on the day where you will be given a seat on another service. Please be aware that our trains are very busy so please arrive as early as you can.
The Eurostar Contact Centre is open 8am to 7pm Monday to Friday and 9am to 5pm on Saturday and Sunday."
As Rojash states, news of any alterations to services always appears on the front page of the Eurostar website.