changing uni course

i am in my final year of a levels, and ive suddenly changed my mind as to what i wantto do, right now ive applied for psych and confirmed my choices, but i changed my mind and i want do do ecology or zoology instead. but the annoying thing is that i chose my a levels for wanting to do psych aka i only did psych as one relating to my course because i dont need more, but for ecology/zoology i neeb bio. my question is 1. do you think it is wise to reapply during my gap year and 2. can i do an a level by myself in one year