I don't think that there is a straightforward answer to this Toadypad. Your box in a while should start prompting for insertion of the new card, but if you don't have it switched on, the old card will not be updated and therefor not receieve the request for insertion of the new card.
The problem may arise when you switch the box on after six weeks, normally the existing card would take a few hours to catch up on the updates, but if Sky are sending out signals to the new card, this may not happen.
Is there nobody there who can do the update for you? It just may save you a lot of later grief.