Purchase Train Tickets Now for Colmar to Annecy and for Avignon to Carcassonne-Barcelona?

The advantage to buying tickets early is your seats will be cheaper and you don’t have to worry about seats selling out: https://www.sncf-connect.com/en-en/.

That is a little misleading. The train pricing structure is a little different from what we are used to from US airlines. Discount tickets are available for most trains but it is limited by numbers and not by time. These tickets have restrictions as to refunds and changes. The number of tickets available per train varies by date and time but it is a fixed number. Once the discount tickets are all sold, you pay the next higher class of fare. On popular trains the tickets will be gone quickly but on a less popular train discount tickets could be available as later as the day before. It just depends. The base fare for a train never changes.

The best place to buy ticket is on the national train site for that country or the ticket machines at the station.

Since your dates are coming up in 1-2 months, I'd buy now. Colmar to Annecy is a long, arduous trip, 7-8 hours with one or two changes. I'd study and shop the best route and train comfort now. Some routes have you taking a local to a TGV, while others have TGVs but you have to go back through Paris. Study your options on SNCF Connect. My site that I've copied for you is in French, so just search for the language button. https://www.sncf-connect.com/

Anignon to Carcassonne is only 3 hours, but again, you have the TER (local trains) with no seat reservations, and Intercité trains that do have reserved seats. For this reason, I'd study it now and buy your tickets.

Carcassonne to Barcelona you will take a local TER to Narbonne and be able to take either a SNCF train from Narbonne to Barcelona or a RENFE (Spanish national train company) train on the same route from Narbonne. The two companies are competing now. You can check the times and prices on the above SNCF Connect site and https://www.renfe.com/es/en .
I have the apps on my phone.

Colmar to Annecy is actually a bit quicker (about 6 hours) through Switzerland, but more expensive and more complicated. Consider it if you are curious about visiting Bern or Geneva along the way.
Otherwise, going through France is not that bad (6h45) thanks to the handful of direct TGVs from Alsace to Lyon.

If you go through Switzerland, a Saver Day Pass (=day pass for all of Switzerland's trains), purchased now, is probably worth it for the Basel-Geneva ticket, but you won't need to buy the local French tickets in advance.

But in most cases I'd advocate going through France, as it is easier.

Thank you all for your assistance. I am not sure I am understanding the train system. When I look for options on the SNCF site, the trains I find go to Nimes-not Narbonne-then Carcassonne. Does this matter? Looking on Nov 5 Avignon to Carcassonne. Any help is appreciated!

To Carcassonne from Avignon, some itineraries have you changing in Narbonne and others in Nîmes. Routes that change in Narbonne are two local trains, neither has reserved seats. Tickets can be bought at any time. The route changing in Nîmes is a local train changing to an Intercités train that does have reserved seats. Those need to be purchased in advance.

Leaving Carcassonne for Barcelona, the route takes you to Narbonne where you change to a high speed train.

When I search on SNCF I only see options to Nimes. Should I also see the TER options to Narbonne? Please share how I need to search or what I am missing! Thanks for your patience!

I have no idea. Two parameters could be in play: I'm in France, you're in N. America, and I'm using the app, while you may be using the website.

But aside from that, if I were traveling to Carcassonne, I would choose the Nîmes option anyway because it puts you on an Intercités train with reserved seats. For me, as I travel around here, I prefer the Intercités over the TER/ local trains, on routes where I have a choice.

Thanks, Bets. I am using the app-probably just need to play around more with it. I appreciate the insider suggestion! I’m going with the Nimes option😊

I finally looked at your date. Indeed, none of the Narbonne options are showing up for that day. They could later. I don’t see any railwork announcements. And no morning trains that day either. I’d grab what I could now with such limited service.

Balso, jolui, any idea about what’s going on?

No clue either, sorry!
SNCF tickets (except Ouigo) are fully refundable these days (some regional trains have a small penalty) so I would secure the tickets as I can. Nov 5 is during school holidays, so it has an impact on availability.

Thank you all for looking into this for me. Going ahead with tickets now as suggested!