For anyone else that is interested, I've just got off the phone with the FAA Airmens Certification Branch.
There
currently is no requirement to replace the cardboard airmens certificate with the new plastic ones, and there is
currently no future date by which you must replace your cardboard certificate.
The cardboard one is still valid indefinately, though that
may change in the future. (Words like, MAY and CURRENTLY were used a lot in the conversation indicating that this is a current issue and subject to change in the future.)
Replacing the cardboard one with the new plastic ones does involve going though the whole verification of foreign licence procedure, even if you went through it already to get the cardboard one. This obviously involves a trip to a FSDO. However it does not require you to return the cardboard one, so you can continue to use that one until the plastic one is obtained.
I hope that is of some help to anyone else wondering about this issue
dp