Great thank you so much for explaining - I might just buy the 2nd class to be safe in this case
It doesn’t seem sold out in 1st on RENFE’s website.
But as there’s no alternative to get seat reservations than buying it at the ticket office in Spain. But you’ll probably won’t be in Spain until another few weeks.
Unless you’ll be touring in Germany earlier, you mlght be able to get a reservation at the international ticket office, when you have all details (train number, etc).
It’s no problem to buy a second class reservation, with a first class ticket.
The other way around, buying a first class seat reservation with a second class ticket is impossible.
1 fits in 2, but 2 not in 1
See this page for other ways to get seat reservations in Spain: