Hi @Kellykin,

In general, unfortunately it neither possible to choose or change your particular TGV seat online, but you may certainly request these services at a ticket office in a train station or by contacting SNCF over the phone. That said, please be advised that it is entirely at the SNCF's decision to grant or reject such requests.

Best of luck with the seat change's request!

In my experience, it doesn’t really matter. Both options have good window positions and you’ll be able to see plenty anyway.

Downstairs is usually quieter and calmer too. Less seats, and no passing passengers, since the connections between the carriages are upstairs.

I doubt if the SNCF will grant you this request, since there are only two daily trains, so demand is heavy.

You can always go to the Bar (in the middle of the train, upstairs) if you want to sit (or stand) upstairs for a bit.

When booking directly on the phone through SNCF I think you can indicate a preference for the upper deck. 

The reservations themselves on this specific train route are now fully refundable (check your ticket to be sure), but the €2 booking fee charged by Eurail/Interrail isn't. So the best approach might be to try to get an upper deck reservation through SNCF, and then cancel these.

I agree with rvdborgt too though, the lower deck is pretty fine too, and have some advantages too.