Originally Posted by
bamba70
I suspect that this may have had something to do with the fact that I bought the ticket via expedia which has a 24 hrs free cancelation option (for an otherwise non-refundable ticket), and tried to apply the upgrade within this time window.
That should have no bearing on whether or not you can apply the upgrade, so long as the ticket had actually been issued (rather than the agent "holding" the booking for 24 hours before auto-cancelling or paying on your behalf).
The explanation you were given by the agent (married segments) is the more logical one.