Why do you prevent converting a building while two buildings are being leveled up? Having the ability to convert, anytime, seems like a good thing for players.
Good question. But I guess it is by game design. By analogy, it is like being unable to use the left-hand drive car while it is being converted to a right-hand drive by the mechanics or technicians. You have to let them finish their job first. And once the vehicle is already set to the right-hand drive, you cannot use it if the same vehicle is being converted back again to a left-hand drive. You have to let the mechanic or technician finish first the conversion.
Yeah, I know. It is a crappy analogy but it was the first thing that came to my mind