Requires a bit more knowledge of what you're trying to do.
If you can identify what has been changed based on the assigned category you could just loop through everything and disconnect it.
But if being connected to that category doesn't automatically mean it was part of this "bulk edit" you either need to create a mark or log (can be just a custom table).
I'm not 100% sure it covers the categorie-page relateion, but might want to look into workflow and/or versioning docs.kentico.com/api11/content-management/page-workflow-and-versioning