I've spent some time this morning looking into this, so I just thought I'd share my findings.
As I understand it, on and upon are indeed interchangeable, the only difference being that upon usually sounds more formal.
I have, however, thought of a few instances where the use of on seems (to me) a bit awkward:
"The storm will soon be on us."While the use of upon here seems (to me) to be the clear choice:
"The storm will soon be upon us."However, perhaps it's simply BECAUSE the latter form is more common, that the former feels awkward.
"The decision is now on us."While the use of upon feels (to me) like an event that has just arrived:
"The decision is now upon us."In this case, the colloquial way a speaker would convey the statement of fact is by placing emphasis on the word us, while a speaker intending to convey that the event has arrived would emphasize the word upon.
The contrast is perhaps most noticeable when the preposition phrase is complement of an adjective:
Adjective + on/upon:
(1) The decision was based on/upon firm evidence.
(2) He was bent on/upon vengeance.
(3) It was incumbent on/upon us.
(4) It looks big on you.
(5) He was really sweet on her.
(6) It was hard on her.
"Upon" occurs as a somewhat formal alternant to "on" with the adjectives in (1-3), but not those in (4-6) where "big" and "sweet" are markedly informal. Other adjectives that prefer "on" include "easy", "keen" and "severe".