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As per the documentation regarding Guest Users for Confluence, you cannot convert users who have a paid license to Guest users.
https://support.atlassian.com/confluence-cloud/docs/invite-guests-for-external-collaboration/Also note that a Guest user would have access to only one Space in Confluence, and you would have to individually add a Guest user to a space. Access to Spaces for Guest users cannot be managed by setting up user groups of Guests and granting access to Spaces based on the groups.
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