If a player makes it through the first round of a tournament, can they play as different corp/runner in the next round or do they have to stick to the same deck all the way through?

In a tournament setting one needs to have 2 (and only 2) decks built (1 Runner; 1 Corp). The player then uses these decks for the duration of said tournament. In the tournaments I have entered, submitted and vetted desk lists were required. Which means, before the first match you submit your deck list to the tournament organizer (TO) or other judge/official. Then the officiating person checks that the deck you have matches the submitted list.

Thanks for clarifying.

Never even thought about playing on that level with any game before but I am liking this one so much I can possibly see me doing it down the line.