I have just purchased train tickets for 2 travelling from Paris to Irun in June and was not given an option to choose or change our seats. Is it possible to do this with an existing reservation? The tickets were booked on the raileurope website.
I am positive there is no possibility to change seats easily (and searching the help on voyages-sncf returned no result). Once you made a reservation, this is not easy to make any change to it. I do not have any reservation right now to try harder to change the seat but recently I cancelled my booking and made a new one just to get a different seat.
So that is your option : cancel or change your booking (if your fare allows it for free) and book a new ticket with a different seat. One issue on top of the fare conditions is that the price may have increased. For trips in France, I recommend you to use the website voyages-sncf. When you made a search for a train, under each trip description, once you select it, there is a button "Choose my place" letting you pick your seat preferences: window, aisle, upper room, lower room, facing forward.
A second option, suggested by this question/answer, is to change seat after you get on board (if they are left free). However, if you both want to change seats (and sit together) that might be tricky. I do not know about Paris-Irun, but on the Paris-Nice line, most seats are booked on the Paris-Marseille section, so changing is almost impossible and if one person decides to switch, it messes up the whole train. Given the length of your trip, at best you would have to change seat every big stop (at Le Mans and Bordeaux at least) to accommodate everyone. Consider also that this has become less and less tolerated to switch seat in TGVs, most French people hate it and do not hide their feelings.
So in the end I consider changing seat as not an option. To be honest, you have few choices: the upper room or lower room (not even on all TGVs) and facing forward. I am not sure the room is a big deal. For facing forward, I think if you ask two other people to switch seat, they are more likely to accept (than for another reason).
I never used raileurope, but on most websites (such as Voyages-sncf's and Eurostar) e-tickets allow you to easily change seats after reservation, while paper tickets usually need to be exchanged physically.
I received good recommendations for Capitaine Train, which includes more seat preferences than Voyages-sncf's. Also, due to the fact that both act as "travel agencies" when booking tickets (i.e., despite the name, SNCF and Voyages-sncf are somewhat "independent entities"), standard ticket prices should be the same in both websites.
On a final note: e-tickets offer several advantages, especially when living abroad, but they also have some issues. For instance, non-refundable e-tickets (e.g. Prem's) cannot be reliably resold (since the owner is in control of the ticket, and could decide to cancel it after the "sale"), while paper tickets can, if they're not personal. It's probably not your situation, but this explains why some people may prefer printed tickets.