You can not use the Eurostar reservation for another train than the train that the reservation is made for. You will not be able to pass the gates without a valid reservation.

Thanks. Totally forgot that they go through very thorough screening beforehand… I guess the ferry it is….

You should be able to change via E* themselves or even make the correct booking for less than a ferry.

Remember a reservation is not linked to a travel day so even if you write off this reservation you still keep the travel day for the actual day you want.

Thanks! Unfortunately, the date I want to leave is full booked. And since a cancellation of the reservation does not give a refund, I doubt there will be spaces opening up closer to the travel date . 

Assuming you have contacted E*and they can't  change it, you could consider a full price ticket for that train or another at a different time.

I know my recent reservation was transferable with a fee. 

Check your ticket.