Admins can Replace a string in all posts.
I don’t think automatic redirecting is a good idea because then visiting the topic containing the category description would no longer work, so you could no longer edit that.
I would probably have simply added a link to the newly created topic in the new text for the category description, and users would then read the short category description first and then follow the link to the other topic.
You could even leave the content in the category description and create a topic with the same text that you pin. But then it could happen that you only edit one of them.