When i was in sixth form you had a maximum of 2 months so end of October if you wanted to switch alevels, which i did (history to econ) it was a great decision as I HATED history, however 8 months in is far too long and chemistry (which i did myself) is one of the hardest content heavy alevels so i would suggest thinking about it carefully alevels are only 2 years long i do not think it is sensible considering you're doing well in geo.