My experience: I was recently waitlisted for a GBR excursion on a Round-Austrailia cruise in 2025. I also reserved and paid for a backup excursion. I never got an email asking to accept the move from waitlist onto reserve, but I did get an email asking to immediately pay the price differential. I paid, got the confirmed reservation for the excursion I really wanted, and the backup excursion was automatically cancelled by Princess.