In a future update they'll add a crafting system, where you will be able to "destroy" extra cards and create cards that you don't have yet, as they say in the official website:
"There’s even more to come as we listen to your feedback, and work towards creating the best game possible by including new features and improving gameplay. Look out for additional features, including real-time competitive PvP and a crafting system, available later this year."
http://www.pvzheroes.com/news-and-media/introducing-plants-vs-zombies-heroes.html
