If you are within the check-in window, click the check-in link first, and then there will be a Change Flight option and it will show you available options. You will get the message you saw if you click the change flight on reservation page instead of going through the check-in process. I do this all the time and have not "lost" my existing reservation in recent times (the last couple of years). It now always asks for confirmation before actually changing your flights. Although you will still run into cases where it says it can't change your flights and you have to call.