The phrase "let upon" has a few different synonyms depending on the context. In some cases, "allow" or "permit" could be used to convey a similar meaning. "Agree to" or "consent to" might also work, especially if the context involves giving someone permission to do something. Alternatively, if the context involves acknowledging or discovering something unexpected, "stumbled upon" or "encountered" could be good synonyms. "Realize" or "recognize" might also work if the meaning is more about suddenly understanding or becoming aware of something. Overall, there are many different ways to express similar ideas to "let upon" depending on the context and desired tone.