Merge/change accounts after SSO

[Judith (Judith Meyer)]

I migrated a phpbb forum to Discourse and then used SSO in order to have everyone log in through our organization’s site, where they already had accounts. This worked great for those who had used the same email address to sign up for the organisation and for the forum, but not great for those who used different email addresses, they all have two accounts now (a legacy one with profile + posts, and a new one from the SSO). Is there any way to change this so that next time they log in via SSO, they find themselves in the legacy account? Ideally from the Admin panel, not commandline, because this will concern hundreds of users. Neither the user email field nor external id field seem editable…