Contrary to popular belief, checking in the moment the 24-hour window opens does not significantly increase your chances of a free upgrade, especially on major airlines. Most modern carriers use automated algorithms to assign upgrades based on a strict hierarchy: first by Elite Status tier, then by fare class (ticket price), and only then by the time of the upgrade request. In fact, checking in too early can sometimes be a disadvantage for "operational upgrades" (OP-UPs). These usually happen at the gate right before boarding if Economy is oversold; the gate agents look for passengers who are already at the airport and haven't boarded yet. However, checking in early is beneficial if the airline offers "Buy-Up" offers. Often, the first people to open the app are shown a discounted cash price to move to First or Business Class before those seats are given away to status holders. So, while "early check-in" won't trick the system into giving you a free seat, it gives you the first "right of refusal" for the most affordable paid upgrades available that day.