There is a simple way to add tags to old posts - just add a level 1 comment with tags or links to objects.

For example, by adding #neuralnetwork tag in this comment, you will link the original post to that tag, and the post will appear in the news feed for that tag.
In addition to tags, you can also use objects (terms, companies, products, places, pages, lists, etc.). For example, you can create an object on Hive for a particular place and then link to it from your posts and comments. All these posts will appear on the page of that object. Here is a tutorial about objects on Hive.

By linking tags or objects from posts, users accumulate expertise, and you can check it in user profiles. Similarly, you can click any object or tag and see the users with the greatest expertise in them.

Maybe this protocol can be useful for the needs of your project as well.