batch books code email essentially expired few file links next page password password-protected pdf pdfs product queue remaining sales sell static stock switch text thank-you thank-you-page threaten unique validity whenever

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?

The best way to do this is to include the "password" in the "thank you" email and "thank you" page. That way, whenever you switch the PDF file, you can switch the static password in the email and thank you page text.