I experienced some major inconvenience months back when I tried to do something similar.

I had just bought a $1200 nonupgradeable fare using two type B vouchers to pay part of it, and 12 hours later an upgradeable fare for almost the same price came up. I wanted to of course get that upgradeable fare instead.

The reservations people insisted
1. that I would have to cancel the ticket using the 24 hour window, no changes were permitted, even though the end result would basically be the same thing if we just made a change.
2. that I could not get the value back for the vouchers I had used, that I would be losing all value.

This was very frustrating, and I gave up trying to convince them. I wrote to customer relations showing them my problem, that I had called in the 24 hour window, and that the fare was still available, and asked them to please fix this issue. They helped me change my ticket, but I don't think the process has improved much...