The deployment team is still in the process of discussing the exact details of how the upgrade will be implemented, but from what I remember, the switch from one version of the app to another will occur for both existing and new users at the same time. There is going to be a new admin event that will make the switch occur. In response to this event, any existing active sessions will have to be serialized, and later deserialized by the classloader of the new version of the app. Of course, the (custom) class definitions of any of the app's session attributes must be backwards compatible across the app's versions for this (and session failover) to (continue to) work.
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