I've tried three times, never been able to do it free of charge. I've never invoked the 20 minute delay rule, though. Something to try next time I guess.

I have, however, tried to switch immediately before the earlier flight is leaving, as smmrfld suggests, with no success. FYI I typically fly on the lowest of the low fare, and I fly a lot, but not enough to be a CP or A-lister.