If an error occurs for a value already existing for a metafield (legacy) but isn't available, navigate to:
/admin/metafields/[type]/migrate/[namespace]/[key]
to migrate to native Shopify metafields editor.
Example for SEO Hidden created with previous methods, route would be:
/admin/metafields/product/migrate/seo/hidden
Other references:
[dev docs] https://shopify.dev/api/examples/marketing-seo#hide-a-resource-from-search-engines-and-sitemaps
[general docs] https://help.shopify.com/en/manual/metafields/metafield-definitions/migrating-metafields-to-a-definition