60 minutes allows you to make a one time change free of charge, provided your origin, destination, and dates of travel remain the same. (This is the message you see on the webpage when you're informed of a schedule change.)
90 minutes allows you to cancel and refund if you'd like. (This is from the Contract of Carriage.)
OP hit the jackpot, and should now be able to switch to any flight that day. If an agent declines, politely thank them, hang up, and call again.