I had seen the following excerpt from this poem in a rhetoric workbook a while ago as an example of a word construction:
They said, "You have a blue guitar, You do not play things as they are."
The man replied, "Things as they are / Are changed upon the blue guitar."
My question is regarding the use of the word "upon" in the last line. Is this usage ever correct, or is this just an example of creative license in poetry?