From this page of our guide, you'll learn how to change Key Cards of your deck.
When it comes to card variety, Gwent: Rogue Mage completely satisfies the player, as there are numerous units, spells, treasure to choose from. Nevertheless, before the start of each expedition, you need to choose a key card that has a unique and very powerful effect and it will define your playstyle.
It should be noted here that key cards aren't affected by Events or Places of Power, but can Evolve after defeating elite opponents. You can only change the key cards before the expedition begins and they will stay in play until the end of that session.