Changing optimized pages with same content

I had started off by trying to optimize all of my product pages. I didn’t make it far before I learned about the benefit of optimizing categories over specific products. I now have a few product pages that are ranked well (top 10) but I would much rather have my overall category pages ranking instead.

My plan is to use similar content I used for the product pages for their respective category pages. My question is regarding what happens to my rankings if Google sees this similar content on two pages (a product page and it’s respective category page).

Will this hurt my rankings for both, only one or none at all?

Should I immediately modify my product pages with different content after “moving” that content to the category pages so it is not seen duplicated (or close to that) by Google on multiple pages or make that change slowly to give things a chance to balance out in the rankings?

I hope the question is clear. I’m having a bit of a hard time verbalizing what I’m thinking… probably just due to my own naivety about how this all works.

Thanks!