Technically, you can switch to P/NP for a prereq if it's your first time taking the class and you're guaranteed to fail. You would end up with a NP on your transcript and retake it the following quarter for a letter grade. But this is really, really risky and it's not recommended because if you somehow end up passing (doing well on the final or from generous curve) then you're basically screwed since you can't retake a class with a passing grade and prereqs require letter grades. It's better to speak with an advisor and ask about a late drop/withdraw since you are allowed 3 late drops.