One of my clients is currently using a different VOD theme, but they want to move to a different service. I was wondering if it would be possible to import existing customers to wpstream, or whether customers would have to sign up again on the new website.
Customer base is not handled by WpStream, rather by WordPress (and WooCommerce if the case). You should be able to just set up WpStream onto your website and use it with existing customers. If you need to move them to a different website, that’s possible for sure, however not a WpStream related topic.