Were there actually any FC seats open before boarding started? If not, the seat was likely open due to a no show and not due to upgrade list not being cleared. The GA is supposed to come aboard and pull up next passenger on list at 15 minute mark in no show situations, but they are also trying to close out flight and at times simply won't bother and let the seat go out empty. One way to handle this if you are number 1 on list is to approach GA before boarding to confirm they will board aircraft and pull you up in the event of any no shows. The FAs aren't really supposed to be upgrading people and you aren't really entitled to anything if they refuse.