you can change from gcse to igcse (international gcse) but they can be a little different. i would look up the igcse equivalents of your gcses and see how similar/different they are and if you need to do extra study for them. if you do igcse then you can do the exams abroad, but they still start and finish at the same exact time as regular british gcses. they do cost money, and they are sometimes expensive, so think about that before deciding which igcses to take. you need to find a local exam centre near where you'll be living and see if you can register to take your exams there.
however, you could fly back to the uk for the exam season and do the gcses at your school. the exam season is two whole months, though so finding accom if you're not staying with family or friends can become expensive.
if you can get accom somewhere in the uk but its not near your school, you can become a 'private candidate' however you'd have to pay to do the exams and it can get fairly expensive once again.
please talk to your parents about your choices because im guessing theyll be paying for the exams and basically all of your choices will require fees. good luck with your desicion and your gcses! xx