Originally Posted by Globaliser
I believe that if you have a Z1-2 period Travelcard plus prepay on your Oyster and you make a trip from Z6 to Z1 (or vice versa), the system should recognise your Z1-2 Travelcard and only deduct the extension fare (Z3-6) from your prepay balance.
That was my take on it. A sort of "we'll figure out what's best for you as you go" price-wise. For frequent London travellers, it seems like a very useful system for those whose plans may change intra-day and would previously end up overpaying.