When you claim loot as XP, the card is stored until you level up. When you have enough total XP across your stored cards, at the end of the encounter you level up. Stored cards with enough XP to achieve the new experience level are removed from the game. Because of this, you might have to remove cards with more XP lanterns in total than the requirement. There is no “change” given or possible - we don’t cut cards in half! You can always press the lantern tracker to see the cards you have stored as XP. Note that the game will automatically only remove cards as necessary, so you can be clever and keep a card sometimes! For example, if the requirement is 8 XP, and you have three 2 XP cards stored, then claiming a 4 XP card as loot will leave you with one 2 XP card stored.