score:6
Unfortunately, my understanding is that the answer is no.
From https://help.loco2.com/article/306-seating-preferences-and-choosing-a-seat
Changing Eurostar seats after booking
The only rail operator who permits a seat change after booking is Eurostar.
While there is nothing to lose by contacting Eurostar directly, Loco2 sell both types of ticket and I would expect their analysis to be correct.
Upvote:5
I agree with Mark Perryman's answer: as far as I know, this is impossible online. However, I have succeeded in getting my seat changed when checking in at the counter with my DB ticket. Of course the choice will be much more limited at this stage, and it may be completely impossible if the train's fully booked. But it's worth trying, especially if you're after some general category (‘window’, ‘near a door’, ‘near the buffet’, etc.) rather than one exact seat. In my case it was ‘My companion and I booked separately, but is there any way you could seat us together?’.