Heavily depends on the route and where you made the reservations but it is rarely possible to change.

First of all it's rare to know the direction of the train, even the company doesn't know.

Eurostar and Thalys can be easily changed.

If you'd like precise advise, let us know the route and where you booked.

Btw unless the train is 100% full, it's not a problem to sit somewhere else than your booked seat. You can also ask nicely other passengers on the train.

In Germany, the planned direction of the ICE trains is known (check e.g. on fernbahn.de) but there's always a fair chance that it runs in the reverse direction...

I tried booking through Interrail and it took me to SNCB to purchase my Eurostar train tickets.  I now have seats assigned but I’m unable to see a seat map. I’ve taken Eurostar in the past and always had a change to see a seat map.  

For Eurostar you can change your seats after booking. Regardless of where you booked, you can go to eurostar.com and then Manage booking.

My bookings are from Barcelona to France and on to Eurostar to London.

I tried to go to the Eurostar website and manage my booking.  It doesn’t recognize my booking reference number.  

It should as it works with mine (I made it at a ticket counter but the ticket is the same).

Maybe try again later if you've just made them.

Can't help about the TGV Barcelona - France

If you have seat preferences, then SNCF bookings are best made by phone via SNCF. If you booked elsewhere, I'm afraid SNCF can't access your booking.

Got to Eurostar and was able to change my seat.  Thank you!!