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You are totally correct. If you are upgrading from 9.0.4 to 9.1.1, you need to uninstall all nodes in your cluster.
For maintenance release upgrades, e.g. 9.0.1 to 9.0.4, you do not need to touch workers; the primary machine will update them automatically.
All other upgrades you must uninstall and reinstall all nodes.
Let me know if you have questions.