Not via app-but thats the last thing to use anyway.
It all depends-welch einer Überraschung-by railway and type of train. You did not say anything about that. Fairly often in some countries this is best done at a counter-SBB should also be able to handle anything in germanspeaking and some around.
And-swiss are bahnly friendly, oder? also kennen die die alte tricks: mal fragen ob jemand tauschen will. And not all seats in all trains are REServed.
Thanks for your quick answer! Our trip is via France to UK (Scotland), so especially TGV Basel - Paris and Eurostar Paris - London and vice versa are in our focus.
I can’t answer for TGV, possibly a SNCF ticket office can change the seats.
On Eurostar as long as there still are free seating bays it should be possible.
Book the additional passenger the same way you already did. Then using the booking details for all 3 reservations on the Eurostar website you should be able to change them to any 3 free seats in the same class.