'Upon' doesn't necessarily replace "based on." It can be used in place of "based on." So, "based on" or "based upon" can both work in the same sentence. For example, "it was 'based upon' (based on) two opposing ideas." 'Upon' (up and on) is used primarily as a preposition. The princess climbed upon her horse and rode off into the sunset. 'Upon' is described as a more formal word for 'on' but that's not always the case. To call 'upon' - to summon. "He was called 'upon' to provide expert testimony during the trial." 'Upon' - a time in the past. Often used in fairy tales. "Once 'upon' a time there was a friendly giant." 'Upon' - immediately after. 'Upon' arriving home, the couple found a stranger inside the garage. Sentences: She carefully placed the vase upon the desk. They built a cottage upon the old hillside. Stealing a child's food is frowned upon. ¡Que tengas un buen dia!

"Upon" is basically just a formal version of "on." We hear it in lectures and speeches and read it in books. We don't typically use it in conversation. It can replace "on" in the phrasal verb "based on," which gives us "based upon," which sounds more formal. Some examples: Once upon a time... (this is the typical way fairytales start in English) This great decision, Mr. President, rests upon your shoulders. Do not disappoint your fellow citizens. "Once upon a midnight dreary, while I pondered, weak and weary, Over many a quaint and curious volume of forgotten lore..." (from The Raven, a poem by Edgar Allan Poe)

Once upon a time .... there was a little dog who went to explore the garden... Upon a little steep there was a flower blooming with all its splendor.