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All reviews are stored in your WordPress database. Unfortunately, the plugin doesn’t (yet) have a utility for exporting reviews from the database. However, the plugin uses the standard WooCommerce format of reviews, so you can use any other third-party plugin (that is compatible with WooCommerce) to export reviews. Try searching the official WordPress plugin directory.
If you are going to completely migrate your site to a new domain, I don’t think that you should do export/import of reviews. Besides reviews, you will still need to migrate all other database tables (with products, customer orders, etc) to your new site. So, it would be easier to just migrate your full database to the new site. This will then copy reviews as well.
