My experience has been standbys are not eligible for upgrades. Under the new rules, one GA patiently explained to me that I cleared standby and to not be upset because if I HAD been on the the upgrade list (list contained 10 passengers) I would have been at the top and would have snagged that last FC seat. But the rules said standbys don't go on the upgrade list.
My many years of travel have taught me to only standby for a later flight if I can live with the possible consequence: not flying at all that day. Only once have I gone standby for the last flight of the day. I didn't clear and I asked myself, "what were you thinking?"