Funny enough, something like this came up in the very first FNM my little shop held. A player was goldfishing with his Vampire deck between rounds, and a spectator thought that he’d switched decks from his Elf deck that was seen round 1.
In a Constructed tournament, no matter what the format is, you pick a deck when you show up and you stick with it. Once you’ve handed in your decklist [if your event uses one], you’re using that deck for the whole event, even if you decide it’s a bad choice later on. You can’t make little changes, you can’t tweak your land base, you have to present your deck exactly as listed on your Decklist at the beginning of each match. You can sideboard after game 1, but only with the 15 cards listed in your decklist’s sideboard. If your deck doesn’t match your list, you’ll get a penalty for that, and have to fix it.
This all holds true even when the event is not using registered decklists (like most tournaments at your local game store). The “decklist” is invisible, but it was set when you presented your deck to your round 1 opponent. Altering your deck from that “list” later in the event is not allowed. If you’re so unhappy with the deck that you don’t want to play with it anymore, simply drop from the event and play some casual games with a different deck instead.
Today’s tournament tip written by Trevor Nuñez