I used this feature to update issues in an existing project. For example I could update existing issues with a different Summary or give them different Story Points. This is not the same as making the same change to a bunch of issues via the Bulk update feature because with External System Import I could update issues to different values without having to edit individually.
Now the External System Import asks me to create a Project but I don't need that; I already have a project 
How do I update issues now then?
Sorry if this is answered elsewhere.
