You could use the CLI/API to export all the users and then edit the resulting JSON file with the new email addresses through the same method. I believe if you try to upload a new CSV for users, it will duplicate them and not change the existing ones. You would need to use the /api/v2/users/{userId} API or gc users list then gc users update in the CLI
The real pain is that the API only accepts one update at a time, so you should probably look at using PostMan to automate the reading of the user ID with the existing email then transforming that and sending the new Email to the same user ID
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Robert Wakefield-Carl
ttec Digital
Sr. Director - Innovation Architects
Robert.WC@ttecdigital.comhttps://www.ttecDigital.comhttps://RobertWC.Blogspot.com------------------------------