change streams can already handle stepdowns and setFCV in a normal upgrade. what makes this kind of upgrade special is we are trying to effectively insert mongos in between the client and mongod. by default change stream cursors would stick around and be cursors directly on mongod still. It would be nice to be able to return a resumable error on this FCV upgrade to make them come back trough the router logic so they are in the same state as if they had been opened on the router to begin with.