Hmmm. Which train exactly (TGV or Intercités?), which date, which time? Where did you buy your seat reservations?

H! TGV. I have a mobile pass and bought seat res on Eurail site. Travelling 28Apr at 1210, TGV 6823.

thanks for your reply!

There's not much availability left (Lyon-Narbonne) for that day I see, so I would not risk rebooking. 

Hmmm, I doubt that the seat would be taken for that stretch (Nîmes Narbonne), as regional trains are much cheaper for that stretch and take only 10 or 15 minutes extra travel time. 

You could do that too btw, add in a stop (maybe a couple of hours) in Nîmes (as long as the TGV stops in Nîmes centre) and continue to Narbonne via reservation free regional train (TER) a bit later. Nîmes is a real treat to visit, even if it's only briefly. I'lld advise you to do this, using the flexibility of the pass. 

15 minutes after the TGV you will be on leaves in Nimes to Montpellier /Narbonne/Toulouse, there's a TER to Narbonne. So very good connection. 

If you do want to try change your reservation, get in touch with Eurail/Interrail via https://eurail.zendesk.com/hc/en-001

But again, wouldn't do that. I'll go for a stop in Nîmes and visit the roman museum there, the arena or the very pleasant gardens. 

You can always ask the ticket inspector if you can stay on the train. The price would have been the same anyway and these trains become emptier if they near the destination, so there's a good chance it will be allowed.

Thank you all for your help!