The dictionary saved the day: MW explains that upon means on, with other definitions considered obsolete.
A few forums took it a step further and explained that upon is considered more formal than on and should be reserved for set expressions (once upon a time...) and more formal settings (legalese!).
In short, if you aren't telling a story and you aren't obsessed with your own intelligence, don't use upon. It doesn't make you sound smarter...it makes you sound stuck-up.