Can a sentence begin with upon? Or should it be changed to on?
- Upon a decrease in temperature... [Is this grammatical?]
- On a decrease in temperature... [What about this?]
There are certain use cases for both, which is why we have both words.
In this example you cannot simply replace upon with on and maintain the same flow.
So why are they different? Upon has a more literary effect to it and sounds pretty formal in general. As FumbleFingers noted upon can indicate a one-time occasion such as
whereas "on" is used less frequently to express one-time occasions.
Judging from the comments I would say it is a stylistic choice, rather than a grammatical choice.