as you current phone is on finance, when you take it back into store (within those 14 days) apple would in essence cancel the loan agreement on your behalf with Barclays. you would then have to start a fresh with the new device and pay however you wanted.
Ah well. Was hoping to keep the finance as was and just exchange paying the difference (only really want to do it because of the camera issues I had). Keep waiting for a 128gb then