We assume schema changes will replicate from master to replicas normally. If you bin/storage upgrade and this isn't true -- replication is stopped or delayed -- Phabricator will fail to come back online because schema changes are missing.

Phabricator's behavior is probably reasonable here, although we could consider weakening the reaction to this situation. Specifically, we could treat missing schema changes on replicas as a setup warning rather than a setup requirement.

In any case, this is a big mess which we should detect and warn about in bin/storage upgrade, before we apply schema changes and make the problem worse and more complicated than it already is.