Tying it all together, if you pass through zone 1 then you will be charged a zone 1 extension fare. You can have more than one travelcard on an Oyster card, but each one has to be for at least two zones and they must join together at adjacent zones. If you just want to add zone 1 then you might be better off waiting. If your travelcard is for a long period then you could try getting an exchange which will cost £5 plus the difference in price for the remainder of the validity. As Alan says, you can add a 16-25 railcard to get discounts on off-peak fares and caps, including extension fares.