Hmm , yes depends on the flight , you will need to check . You can try now , if not possible you need to wait at checkin . When checkin will open (h-30 from first segment of your journey) , either you will receive an email with a proposal , or you might see a popup , or if no offer , then try to select your seat and you will see what is available

last year i was in Y , and when i tried to check the price of J from the app i was able to "upgrade myself" for 460€/550$there is a thread of this sub-forum dedicated to upgrades , people report how much they paid or been asked , and at which step of the process = The ultimate thread on buy-ups at OLCI , unfortunately for you FIH/BZV are not very popular routes here , so dont think anyone posted about it