Reading this article: https://support.atlassian.com/migration/docs/migrate-jira-service-management-customers/
One outstanding question I have is about the password being migrated.
When we tested this migration by migrating into a sandbox environment I was not able to log into the portal with my customer only account.
I am wondering if this is because I am in the sandbox environment or I would experience the same issue when we migrate to production.
Hi Jeffrey, customer account passwords are not included in the migration.
When we migrated from Server to Cloud I instructed all our (non-SSO) customer accounts to perform a password reset from the Cloud portal.
Also refer to the following article: https://support.atlassian.com/migration/docs/get-your-users-started-in-cloud-after-migrating/