I would like to suggest a rethink about the length of time required to upgrade buildings. A Level 1 to Level 2 upgrade is reasonably quick. After that, the situation deteriorates very badly:
Level 1 to Level 2 - 2 minutes 30 seconds
Level 2 to Level 3 - 60 minutes
Level 3 to Level 4 - 24 hours
And during an upgrade, no production takes place.
How long is a Level 4 to level 5 upgrade?? I have no idea, but if it's consistent with the sequence above, then it'll be 24 days. And quite frankly, in this game, I wouldn't be surprised if it does take that long.
Why are the higher level upgrades so extremely slow? Please reconsider whether it's really necessary to impose such a penalty on players who are trying to make progress without lashing out on extra building licenses.