@evanm Yes you can use the sub inside of the JWT when making any API call to your backend. In your database this will be a unique Id for the user. You can even get more details about the user from FusionAuth by calling the User endpoints.
This exception is caused by the fact that your objects in elastic now have two different schemas (one with the number, one with the string). You need to manually fix that; one way to proceed might be to access elastic directly and remove that one user.
I upgraded. I haven't tried a new install nor do I want to. I understand why it's happening. Is it something you can fix in a future update, without my having to start over with a whole new install?