As a Zendesk administrator, when I enable single sign-on on my account, what affect does this have on existing users?
What happens to existing users depends on what email address is listed on their Zendesk profile. If the primary email address on the existing user's profile is the same as their email in the single sign-on system, the user will have no trouble logging in and accessing their profile.
If the primary email address on the current user's profile is different from their email in the single sign-on system, a new user profile is created for their new email address.
If you do find a duplicate account is accidentally created for an existing user, you can always merge the two profiles together as described in this article: Merging a user's duplicate account.