Where/how did you book these reservations?
There should be a reference to the applicable conditions in the confirmation email you received (if you booked online).
Belgian railways’ conditions say you can change a Eurostar reservation once, before the train departure, which will cost 15 EUR (20 in 1st class) per seat. Conditions are not necessarily the same for every reseller but they're likely to be similar.
Hi,
I bought them through the system on the interrail page.
London - Paris, Paris - London trains
So it seems I could change the reservation through Eurostar, is it still valid with my interrail pass even though the change was made through Eurostar’s website?
So it seems I could change the reservation through Eurostar, is it still valid with my interrail pass even though the change was made through Eurostar’s website?
If you have to change it through Eurostar (and pay 15 EUR/seat there, I suppose), then that's the way to go. Your reservation will remain valid. Remember that the pass and the reservations are completely separate. You can buy reservations from many places and there's no obligation to do that via the Interrail website.