How to remove category slug from old pages, while adding it to new ones?

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I’ve tried googling the answer but can’t figure out how to do it.

I would like to structure my website by topics, for example roses and tulips beings the parent topics:
site.com/roses/history-of-roses/
site.com/tulips/tulip-bouquets/
New posts structure is perfect, the problem is I have old posts which have good backlinks towards them, and when I edit those pages, the URL changes and has a category slug added to it, and I don’t want to do that.

Is there a way to not change old post URL structure when editing, yet keep the site.com/%category%/%postname%/ structure for new posts?

Thanks in advance WordPress pros!

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