Can you sideboard a main Champion unit?

Yes, you can include Champion units in your sideboard.

According to Rule 403.4.a, during the sideboarding process between games of a match, a player may change their Chosen Champion to one from their sideboard or Main Deck, provided that the new Chosen Champion meets the deckbuilding rules of the competition format.

Additionally, Rule 403.3 notes that limits on copies of named cards apply to the combination of your Main Deck and sideboard. Therefore, as long as your total count of a specific Champion unit across your Main Deck and sideboard does not exceed the allowed limit (typically 3 copies), it is legal to include them in your sideboard.