Michael Hoffman wrote:

There is a potential problem with the Student Oysters (and others) - from
the 2005/2006 academic year TfL switched from a photo card and a separate
Oyster (which could be transferred to the new year's photocard easily) to
issuing the annual ticket on an Oyster with the photo printed on it.
Whilst the Oyster can still be used after the end of the year (with the
discount deactivated) it doesn't appear to be possible to transfer money
from the old card to the new. As one who uses the travelcard almost
exclusively and doesn't go beyond Moor Park *that* often (and almost
never travelling exclusively up there), I have very limited options for
using up the credit on the old cards.

You can't get a refund on your old credit?

No. Each time I've registered the new card and brought a season ticket I've
asked about transferring over the balance (and because it's touch and go as
to when the new card comes you have to have balance on there for the
interim) and been told it's not possible.