PyPDF2 currently writes to v1.3 (Acrobat 4.x). Is it possible to allow saving to other versions?

We recently used the library to combine several thousands of pages into file sets (of engineering drawings). Each of the individual filenames (pages) were used as bookmarks in the set. I have found that I cannot use Acrobat X to insert or replace a page manually afterwards because it messes up the bookmarks. If I save the modified PDF using the optimized pdf option (where I can specify the version I want and in this case it would be 1.3) the bookmarks are deleted.

Before:

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After: