It's an interesting question. As the link below indicates, they are generally interchangeable, however, with the exception of definition 4 (a fairly limited situation). https://www.collinsdictionary.com/dictionary/english/upon If you are in doubt, use "on". If you see "upon" used in formal texts written by native speakers, then feel free to imitate the usage. For example, "depend upon" is quite common. If you have never seen "upon" replace "on", then don't risk it. For example, we never say, "I found it upon the internet", "She is upon the phone", "I went there upon foot".