AppZoic

Posts Importer is a powerful WordPress plugin that allows you to import posts from any WordPress website that has the REST API enabled. The plugin seamlessly imports posts along with their categories, tags, and featured images while maintaining proper WordPress security standards.
Key Features:
<ul>
<li>REST API Integration: Connect to any WordPress site’s REST API</li>
<li>Complete Post Import: Import posts with titles, content, and excerpts</li>
<li>Category Preservation: Automatically imports and creates categories</li>
<li>Tag Management: Imports and creates tags from source posts</li>
<li>Featured Image Support: Downloads and sets featured images</li>
<li>Pagination Support: Browse through multiple pages of posts</li>
<li>Security First: Built with WordPress security best practices</li>
<li>Duplicate Detection: Prevents importing the same post multiple times</li>
<li>User-Friendly Interface: Clean admin interface for easy management</li>
</ul>
Use Cases: 
– Migrate content from one WordPress site to another 
– Aggregate posts from multiple WordPress sites 
– Content syndication between related websites 
– Development and staging site content synchronization
<h3>License</h3>
This plugin is licensed under the GPL v2 or later.

REST API Posts Importer

Import posts from any WordPress site's REST API with categories, tags, and featured images.

WordPress.org

Convert existing categories to tags or tags to categories, selectively.

Categories to Tags Converter

Th plugin is old but still work perfectly. I have tried it on a modern install I have, with PHP7.

Works fine, with PHP 7 too

Convert existing categories to tags or tags to categories, selectively.

REST API Posts Importer vs Categories to Tags Converter