Which country and route / date?
I heard that in some countries it is possible but not in all.

France, from Avignon to Lyon Part Dieu (30th May)

Why not taking a TER train without a reservation for this trip? Takes a bit longer but you’ll save the reservation fee. 
There are frequent train connections from Avignon Centre station to Lyon.

In other case: according to a German railway forum it should be possible to upgrade for a fee of 25-30 euro to 1st class in France. But I’ve never done this by myself and suppose you can only do this at a station in France. Maybe other users here know more about this

Thanks for your advice! Unfortunately TER would be too slow. I have to catch a high speed train in Lyon… I’ll wait and see, if other users have more information about the 25-30 eur upgrade you are mentioning.

You didn't mention the departure time, which train in Lyon you want to catch or which Avignon station you'd like to depart from... but generally, for French TGVs you can book a 1st class reservation with a pass for 2nd class. It's just a bit more expensive. You can book that:

• Via this Belgian railways website (booking fee 4€ per order)
• Via SNCF by phone (press #85 for English, no booking fee)
• Via Belgian railways by phone (no booking fee, but the French number should be cheaper).
• Ticket offices in Belgium and France

Keep in mind that the Interrail website currently does not show the 20€ TGV reservations that are available after the 10€ ones have sold out. Belgian railways (see link above) will also show the 20€ seats, which are available until the train is full.

Also keep in mind that there are 2 stations in Avignon. Avignon Centre is close to the city centre but Avignon TGV is just south of the city and it will take time to get there (there are trains).

Missed the morning Paris-Milan train 2020, went to the booking office but the afternoon train was fully booked in 2nd class, payed the extra fee for a 1st class reservation on a 2nd class pass.

Good to know! Thank you very much!

How do I unlock my tickets?

@Braian Cuiña 

Please make a new post with you question and describe the problem you have in more detail.