I agree with Ed and Sven. ON and UPON are essentially synonymous. Here is a dictionary definition: UPON is a more formal term for ON, especially in abstract senses. ("it was based upon two principles"). Sometimes people choose one or the other because the sentence will sound better or flow more rhythmically when spoken aloud, but I don't see a difference in meaning. Perhaps, the definition cited suggests that ON refers to more tangible evidence and UPON to more abstract evidence. In this sense, "based ON the presence of a bullet wound, he concluded that a murder had been committed" BUT "based UPON the theory of relativity, we cannot travel faster than the speed of light". Even here, I think on and upon are interchangeable.

Camilo- I agree with Ed. I cant think of a situation where either could be used. "upon" is a little more forma, but would be the word most used in academic writings. But "on" would still be fine and accurate.

As a native speaker, there is no reason why "on" isn't right. "Upon" sounds a bit more formal, but it's not necessarily needed. Ed English tutor from USA