set all new and existing subscribers to standard membership

Is there a way for all new and existing subscribers to my website to be converted into standard membership? I am trying to protect certain content in my site using the shortcode and it's working fine, except all my user registrations are not able to see the protected content even when logged in because their membership is not set to standard membership. Is there a way for them to be automatically converted from mere "subscribers" to standard members? Hope my question makes sense.