If you are building decks in the deck builder it automatically won’t let you put 2 champions in one deck.
That’s good, however it also prevents you from replacing a champ with another champ, as it thinks there is still a champ in the deck so it can’t add another one.
It would just be an optimal change to make champs be able to get replaced by other champs without having to remove the champ before adding the new champ.