The Taxonomy Switcher plugin allows you to select your “From”, and “To” Taxonomies, to convert all terms. Optionally select a parent term to limit terms to switch. Typing term names will do a live search of terms with the name you are typing AND possess child terms.
Taxonomy Switcher also has built-in support for WP-CLI. In the command line, type in wp taxonomy-switcher for instructions.
Pull Requests and Forks welcome on GitHub
Notes
Please keep in mind, if parent isn’t set, or you don’t specify a comma-separated list of term ids to migrate, it will migrate all terms for that taxonomy to the new taxonomy.