We sell books that are password-protected PDFs, although there is not a unique password for each one. Essentially, every 20 or so sales, we'd like to be able to change out the PDF (which uses a different password). Is it possible to queue up the next batch of 20 while still having a few of the old stock remaining? It is essentially the same product, just with a different code. If that is not possible, then when I change out the PDF, will that threaten the validity of the links recently sent out that have not expired?