yup WardyGTC is correct, upon surrendering a vehicle you get your AP back in pro-rata form, the GCB is forfeited.
Exactly this happened to me – surrendered car as too small, once new car is available for collection I will receive pro-rate on the £999 I paid for current car (losing approx £27pm until new car drops)
GCB is paid on completion of 36-60 month contract so that makes sense