Upgrading may be best served a few weeks before you actually need the seats, just in case you get the construction crew that goes on spring break with the job half done.
I think that's why Buzzerbeater doesn't let you know the exact time of the upgrade: because you can't upgrade for a specific game if you don't know the upgrade will be done in time. They're trying to dissuade people from expanding just because they've got certain games coming up.
Personally, I just assume all upgrades take seven days more than stated. That way I can be pleasantly surprised when they complete "early."