When and how can I change my chosen champion between games in a best of 3, and does it need to come from my sideboard?

Ruling: You can change your chosen champion during sideboarding to any unit that satisfies the chosen champion requirements in your main deck after sideboarding is complete. The champion does not need to come from your sideboard.

Sequence:

• Sideboarding occurs
• Choose your champion from any valid unit in your post-sideboard main deck
• Reveal chosen champions
• Select battlefields