Can you sideboard before the start of the match after seeing the opponent legend/champion unit?

No, you cannot sideboard before the start of the match after seeing your opponent's Legend or Champion unit.

According to Rule 403.5, players may not use sideboard cards in the first game of a match. Sideboarding is only permitted between games of a match (Rule 403.4).

Additionally, per Rule 406.1.e, the start-of-game procedure—which includes placing your Legend and Chosen Champion—occurs as part of the initial setup, and you are not permitted to use sideboard cards at this stage.