As part of an upgrade of a cluster from v2.2 to v2.4, the config database will need to be upgraded and epochs added to all collections (to support SERVER-939). v2.2 is backwards-compatible with these changes, however v2.0 is not. In order to migrate the cluster, a single v2.4 mongos must be started with --upgrade, and no collection metadata changes (adding shards, dropping databases, dropping collections) can happen while the upgrade is in progress.

We need to doc this upgrade process for 2.3.2/2.4.