Does my Chosen Champion need to already be in my Main Deck to swap it in from the sideboard between games?

Per rule 403.4.a, between games of a match you may change your Chosen Champion to one from your sideboard or Main Deck, as long as it meets the deckbuilding rules of the competition format. It does not need to already be in your Main Deck — a copy in your sideboard is enough, as long as you exchange it 1-for-1 with a card from your Main Deck and your deck still meets its size requirements afterward.