Do the parent umpires swap roles during the game?

Yes. Association umpires aren't guaranteed in Stage 1, so each team provides a volunteer parent umpire. Either each team's umpire does square leg for half the match, or each umpire does the main (bowler's end) role for one half and square leg for the other. Agree with the other team's umpire before the first ball which way you're doing it.