Hello!

Yes, it is enough just to change prefix in BxMessengerConfig.php file, in this case the all new Rooms will have new prefix, but already created will have old prefix. If you want to remove UNA_ for existed rooms also, you need to edit table records in bx_messenger_jvc (file - room).  If you have no real talks yet, you just can truncate the table bx_messenger_jvc