I was concerned about my psychology exam as this was what i wanted to study at Uni and if i didnt get a B i would need to resit and i was very nervous about doing so as everything has now changed with the 2 year system as you said. My teachers at college told me that most exam boards have 'Legacy' papers which exist after the examination changes for syllabuses and things like this big change within college. For this reason I would have to believe that your exam board would have 1-2 years worth of legacy papers that you could sit to get a better grade which focus on the same modules you learned in your A level studies. I cannot imagine you will be forced to resit a whole two years for one subject.