It appears that Travelcards will be on sale from the station. Here is a clarification from TfL:

"Passengers will be able to renew Oyster Travelcards at selected National Rail stations. At stations recently taken over by London Overground - New Cross Gate to West Croydon - passengers will be able to purchase weekly or monthly Travelcards on Oyster."

However, don't all rush down to the station to renew your Travelcard. Ticketing machinery has yet to be supplied to local stations and so you will not be able to renew cards until "sometime after January"!

Now admittedly, you can buy Oyster online, by post, telephone and from 3,700 outlets across London. But is it really too much to ask that rail authorities could get their act together to ensure that you can actually purchase Oystercards at stations to coincide with their introduction?!?