Need to create a variation of a policy manual. I need to keep BOTH editions.

Please bear with me. I'm fairly new to InDesign and learning on the job. I have created a book file with chapters (policy manual). I now need to create a modified edition of the policies in addition to the original. Most of the information will be the same and I want it to be formatted the same. Also, this is important because we update every couple of years and I need to keep each edition intact for archival purposes.

I ask because when I first started learning InDesign I was working on a book with chapters and did some editing on one of the chapters as an experiment then saved the chapter with a different name in a different folder and then when I went back to the book file, to continue writing and the chapter I had "experimented" on was showing as needing to be synced or updated. I didn't want to use my experiment so I didn't want to update. I hope my question makes sense.