Usually you have to pay then ontop of the reservation the class upgrade Difference between normal fare 2nd class and normal fare 1st class :/

Thank u. How do we know what the difference would be? 

Ask at the station. Indeed, in Spain you can often buy a 1st class reservation with a 2nd class pass but I don't know how they determine the prices for it. It will surely be more expensive than a normal second class reservation.

Thank u :)

There are just 2 direct fast trains between thse.

There are nearly hourly CONNections via Madrid-which thus means you have to change trains, book 2 trains and it takes longer.

These very long distance train is very often booked solid-many reports about that.

You can cehck the price if you are able to work via renfe.com and check prices for your day and then the difference between the classes (which RENFE calls turista and preferente-there is also a much more expensive bisnis class)

Thank u. We will check