Term Taxonomy Converter
Initial version of this plugin is a fork of https://wordpress.org/plugins/taxonomy-converter/.
Copy or convert terms between taxonomies.
Taxonomies include categories, tags and any other custom taxonomies that may be present with your active theme and plugins.
This plugin, allows you to copy (duplicate) or convert (move) terms from one taxonomy to another or to multiple taxonomies, while maintaining associated posts.
Large sites
Conversions run in batches over AJAX with a progress bar, so large term sets no longer time out. A Preview (dry run) button reports exactly what will happen — how many terms would be created, how many posts re-tagged, and how many source terms deleted — before you commit.
WP-CLI
For scripted or very large migrations, use the command line:
# Preview copying two categories into post_tag
wp term-tax-converter convert 12 15 --from=category --to=post_tag --dry-run
# Convert (move) all categories into a custom taxonomy
wp term-tax-converter convert --all --from=category --to=topic --mode=convert
Run wp help term-tax-converter convert for all options.