V2 Change stream readers rely on a distributed protocol that shard and config server nodes conditionally honor based on the value of the FCV gated featureFlagChangeStreamPreciseShardTargeting - and are expected to fall back to the previous behavior as soon as they can detect that the cluster has entered a downgrade process.

Such a transition can be easily observed from mongos when (re)targeting requests are sent to the config server (the first replica set to switch its FCV state when a cluster downgrade is initiated): these requests are expexted be replied with a  NotAvailable state when the feature flag is disabled.