Normally you have 15 minutes to claim your reservation. If you don't do that the seat can be given to someone else. But I don't know how this rule is used on the trains between France and Spain.

These trains only make very few stops-2 further on is Avignon or even more on-not really a short distance. So you must have chnaged plans a lot.

What anna tell s as such is true, but will depend on the conductor checking and reporting that seat has not been taken-as all seats are RES anyway, this is not very likely that someone turns up at very last minute. More likely is that the person beside it has decided its free and put her luggages on it-or told a friend seated far away to come over. Claiming UNres seats is more useful in DE/AT in full trains with standees.

So what would you recommend going to Lyon or try to get in the train one stop later? 

If you don't go to Lyon you risk loosing your seat. But that's your own choice. Where did you plan to catch the train?

i want to get in to the train at Nimes. i also talked to someone on the phone from SNCF they Said if its one stop later it shouldn't be a Problem