I’m thinking of “retiring” an older publication and trying KDP Select for a period. While retired, can existing book owners still download the book again if they wanted?

Yes, existing book owners can still download the book from their Leanpub library, after the book has been retired.

To 'retire' your book from Leanpub, go to the Book Tools menu for your book, select Admin , and the click on Retire Book . Then, click the red "Retire my book" button to retire your book.

You can unretire your book later, if you decide to start selling your book on Leanpub again.