I'm not sure the last agent spoke the truth, as normally one cannot waitlist (or, for that matter, stand by) for an upgrade class unless one is already confirmed on the flight.

No more than one can directly book into an upgrade inventory; you must first book the revenue inventory, then process the upgrade.

And I would presume that, if she is right, and if you do clear a waitlist for one of the other flights, the airline will reprice the ticket at that time using prices in effect that day for the flight you're going to actually take, plus if the original ticket was restricted you'll get hit with a $200 change fee as well.

If you're booking a Y fare for the trip, then it should just be an even exchange.