RFD 289 describes a few pieces required for Steno upgrade:

• Nexus should only ever run sagas created by the same version of the software. In the absence of failover for saga execution coordinator #2317, and if we never upgrade Nexus instances in-place, then maybe there's not much we need to do here? Because Nexus today should only recover sagas that it created, and a given instance will always be on one version.
• Sagas need to be drained from Nexus instances during upgrade.