Needed some time to actually look at this and trying to figure it out. And to write up what I have tried and failed with.
I don’t understand what “changing it at the source” means. The SSO database already has a different email but it does not automatically propagate to Discourse when you log in. I actually published the code I’m using ages ago here:
https://meta.discourse.org/t/discourse-sso-external-site-provider-problems/32977/12
Somewhere in the middle where nonce is created, maybe there is some parameter I could add that would say “force_email_override=yes”.
But changing code for such a simple thing feels weird so I’ve tried the following using the site.
I’ve changed
“email editable - Allow users to change their e-mail address after registration.”
and that makes no difference.
So, I’ve tried letting an admin change the email (email editable both on and off).
There is no edit possibility next to Show. Showing it shows no edit. Looking at the public profile doesn’t show any way to edit it. Stumped 
.
I’ve tried impersonating… nope.
I’ve tried logging in as the user with “email editable” on and off. The user cannot edit the email either.
Running out of options to try… perhaps I need to run all the way via a command prompt and ./launcher to force the “email editable” to take effect?
Getting to the point when I’ve tried so many different approaches I probably have overlooked something basic by now…