http://docs.mongodb.org/master/release-notes/2.6-upgrade
Apart from downgrading Authentication model, Text index and 2dsphere index, the downgrade order should be opposite with the upgrade order, so it should be:
- Downgrade each shard using rolling downgrade.
- Downgrade the config servers. The first config server in the mongos --configdb argument should be downgraded last.
- Downgrade the mongos. It looks like there is no metadata change from v4 to v5, so the downgrade should be just a binary drop-in replacement.
It would be nice if we can document these steps.