Training can be performed on cards marked with a green arrow in the upper-right corner. This is a one-time morph, and can either result in a straight upgrade or a choice between two different morphs. When you train a card, one of the copies of the trained card is converted. When you have the opportunity to train a card, you cannot postpone or defer doing so. Once you have no copies of the original card after it is trained, it will no longer appear in your collection.