I'm not sure of the exact threshold, but I'd be surprised if it was as low as 60%. My understanding is that AC does send customers an e-mail around T-24 hours if the flight is expected to be above a certain threshold. I received one email for a flight that was 100% full at boarding, and another that went out about 95% full after the e-mail offer to change flights. So I think the current policy is that you're only eligible to change if you receive the "your flight is fairly full" e-mail the day before your flight.