That depends. Many airlines offer upgrades at the airport if there are seats available before boarding. However, this doesn't mean that those seats will be just given away. Depending on the route, you can expect the carrier to charge somewhere between €500 and €1,000 per oneway flight if you want to upgrade to business class. In that case, it could be much more reasonable to purchase a business class ticket in the first place. Also, you know that you'll fly in style for sure. This is great since, after all: Anticipation is the greatest joy.