What happens to the existing product keys?
Currently existing product keys get empty license field values (even if the new license field is "required"). Of course you can go and edit the existing keys with new values.
In the very near future we're improving this behavior. That is, we'll let you set default value for existing keys at the time of the new license field creation.
Are they automatically updated with new fields when their program calls IsGenuineEx?
Yes, new license fields that have values are updated when IsGenuineEx() contacts our servers. See: Changing fields & getting the changed data in your app.