Yes you can use your booking code from your booking to add it either on the SNCB Page or Eurostar Page and then manage  it If there are Eurail/Interrai seats left you can change it for 15€ :) 

Omg ! Thank you so much! Ive just gone online to the Eurostar site using my booking reference and changed it to the day before. Phew, thought I was down $60 AUD! 

@seewulf … Wondering if you know if the changes I made on the Eurostar page eventually change on my Eurail account? Or will this just stay the same

Eurail have no way of knowing these changes so no this will not change. You should have the updated reservations from Eurostar, this is all you need, with your valid pass to travel on the updated train.

excatly as said above Eurail wont know your change and even you wont find your updated reservation in the Eurail account. Bestway is to add your new Eurostar Reservation to one of the Apps (SNCB International or Eurostars own or add it to your digital wallet.)