scribu

Custom fields are great for storing various meta data about a post. However, if you routinely need to display posts that have a certain custom field value, you should consider switching to a <a href="https://codex.wordpress.org/Custom_Taxonomies" rel="nofollow ugc">custom taxonomy</a>, which is better suited for grouping posts.
With this simple plugin, you can convert all custom fields with a certain key to terms in a certain taxonomy, while maintaining the post association.
Note: If you’re looking for the old version, see <a href="https://wordpress.org/extend/plugins/custom-query-fields/" rel="ugc">Custom Query Fields</a>
Links: <a href="http://scribu.net/wordpress/custom-field-taxonomies" rel="nofollow ugc">Plugin News</a> | <a href="http://scribu.net" rel="nofollow ugc">Author’s Site</a>

Custom Field Taxonomies

Convert custom fields to tags, categories or taxonomy terms

Andrew Norcross

tripflex

Creates a panel to change or delete meta keys in bulk. Useful for when you are switching plugins or themes that use specific meta keys for functionality, or for general cleanup for older sites that may have older meta data that is no longer in use.

Post Meta Manager

A simple utility plugin for changing or deleting post or user meta (custom fields) keys in bulk.

Custom Field Taxonomies vs Post Meta Manager