Well, there's the rules and there's what the agent will do for you. I found that pre-merger I could almost always get on an earlier flight if there was a seat available by calling the elite line. Now most of my itineraries, which connect through IAH, contain a Skywest/United Express segment, can't be changed because the original fare class rule is enforced and fare classes never open on the United segments (even at three hours before, which is when the agent says they usually do). Three hours before is of limited utility anyway since that means hanging by the phone with bags packed and calling at T-3; the earlier flight is almost never worth that to me (or hanging out at the airport hoping for a standby seat).