I was wondering if there is a way to upgrade them manually... i mean, my largest city profits a lot already even with low taxes (20k simoleons each month and 13M in reserve, mainly due to a top notch public transportation). So i have a good margin to upgrade my Small Airport to a medium or large one, because why not? (There are some airports that can be modded making them pretty huge, but i dont wanna deal with them atm)