it's definitely possible to go from a U to a C in AS chemistry. last year i was failing BADLY in chemistry. i think a month or two before my end of yr12 exams, i realised that i might get dropped from chem if i continue with a U. so i talked to my teacher, did a bunch of booster lessons, actually LISTENED in class instead of daydreaming and did a lot of revision and managed to scrape a C in the mocks. currently in yr 13 trying to push that C into a hopeful B in these a-levels lol