For existing users on WordPress who do not yet have an account on Discourse, they will be able to login to Discourse without any issue.
For existing users on Discourse who do not yet have an account on WordPress, they will need to create a WordPress account so that they can login to Discourse with SSO. If they create a WordPress account using the same email address that they use on Discourse, they should be able to login to their existing Discourse account. If you are not using the default WordPress registration system, there will be a problem when users with existing Discourse accounts initially try to login from WordPress. If this is the case, there are a few ways it can be sorted out. For reference, if you are using the default WordPress registration system, registration will look something like this:
If this is not the way users are registering, the issue is that the WP Discourse plugin has no way of knowing that users email addresses are valid, so it sends a require_activation parameter with the SSO payload. When require_activation is set to true, Discourse will not match existing users by their email address.
Users who had accounts on both WordPress and Discourse prior to your having enabled the WP Discourse plugin will be able to login to Discourse via WordPress without issues. If they have used the same email address on WordPress and Discourse, they will be logged into their existing Discourse account. If they are using different email addresses on the two systems, a new Discourse account will be created based on the user’s WordPress email address.