A PCP is only another form of HP, so you can change you car at any point during the term - the finance just has to be settled (with an early settlement discount because of a reduction in interest payable)and the contract is finished. It can be settled by a VW dealer or any other dealer, so you aren't even limited to another VW! The settlement will come out of the PX price, with any excess acting as a deposit for your next car.
Once half the total amount has been paid, you also have the option to hand the car back to VWFS and walk away - it's usually tucked away in the small print but often not mentioned! I did this with my last car as the residual value was lower than the 50% still outstanding - i.e 'negative equity' in effect.