If I file a revision to a provisional patent (some diagrams are not detailed enough), would the original diagrams be automatically destroyed / kept secret, or will they still show?
I'm a US practitioner, so what I say goes for the USPTO:
Anything submitted in the file of a published application, even material that's been made obsolete by revision or amendment, is open to the public.
Provisional applications are not published, so if you do nothing and let the provisional application expire, then the drawings will remain confidential.
If you file a non-provisional application that claims priority to the provisional application, to keep the drawings confidential, you must submit a non-publication request in the non-provisional application.
If your unpublished, non-provisional application is allowed, then the only way to keep the drawings confidential is to petition that the drawings be expunged. See MPEP 724.05. This is not a common situation, and the Office may deny the request.