I had installed mod_certificate (https://moodle.org/plugins/mod_certificate) at my moodle 3.2, but after upgrade to 3.9 I realized the plugin and all certificates have disapeared. If I install the mod_customcert (https://moodle.org/plugins/mod_customcert), is there a way to migrate the old certificates to make them available again with this last plugin, or they are completed lost?
Nothing should be deleted from your site. If you have access to your database check that the certificate and certificate_issues tables exist. If they do your data is still there. I haven't used mod_certificate in a long time but I dont see why an upgrade would remove them