MongoDB version 3.2 introduces the ability to run Replica Set Config Servers (also referred to as "config servers as replica sets", or running in CSRS mode). This feature provides stronger consistency guarantees, easier management, better support for multi-datacenter deployments, and the ability to run more than 3 config servers.
Upgrading the config servers to be replica sets is recommended for all users running MongoDB version 3.2, and will be required before moving to the forthcoming version 3.4 release. The legacy 'mirrored' configuration for replica sets will not be supported starting in MongoDB version 3.4.
While it was possible to upgrade existing deployments from the old 'mirrored' deployment model to CSRS in versions MongoDB versions 3.2.0 to 3.2.3, doing so required taking cluster-wide downtime. Starting in version 3.2.4 it is possible to upgrade your existing sharded cluster to CSRS mode without incurring downtime.
To start using Replica Set Config Servers in your MongoDB 3.2 cluster please see the upgrade documentation.