I’ve recently booked 2 x London St Pancras - Briançon. This journey involves a change to the night train in Paris. I’ve booked a 6 berth couchette because when I tried to upgrade to a 4 berth it was upgrading my Eurostar leg as well. This made the ticket too pricey for me. So my question is, does anyone know if you can upgrade the SNCF leg of the journey only? The ticket is already booked.