The above is not completely accurate. While you may end up in a middle seat the system does not assign the seat. You have to choose the seat from those remaining in MCS. The sooner you check on the upgrade, the more likely you will not end up in a middle seat. Getting it assigned at the airport almost guarantees a middle seat unless the flight is very empty.

If you wait until the airport to get on the list, the chance of an upgrade would be greatly diminished. Many people are being given gold/silver status through matches. All the gold and silvers already on the list may have been upgraded. Add in those that paid for an upgrade during online check in process, and the chances for an upgrade go down even more.