During that time, a player may change their Chosen Champion to one from their sideboard or Main Deck that meets the deckbuilding rules of the competition format.
§ 403.4.a Tournament Rules PDF (officieel)
§ 403 Sideboard
§ 403.4 Players may use sideboard cards to modify their deck in between games of a match. Sideboard cards must be exchanged 1 for 1 with Main Deck cards.
