These words have a shared meaning,"in the near future." One can say,"An early departure time has been scheduled for the flight" or "The flight will arrive soon." Something referred to as early, however, also means something that comes or appears before the appointed or scheduled time: "One plane arrived early and had to wait for the other." Soon means "within a brief period after a specified time or event": "Once the second plane arrived, welcoming ceremonies soon began." Early on, a British expression that has become popular in the United States, is now a trite phrase. It is also a wordy one. Omit the on and say early, soon, quickly, or immediately.
Dictionary of problem words and expressions. Harry Shaw. 1975.