If you upgrade from SCCC to CSRS, but the shard doesn't have to talk to the config servers for a long time (because no splits or migrations are happening on that shard), the shard may not discover that the config servers have switched to CSRS mode. This can cause problems if you ever want to retire the last former SCCC config server. We can mitigate this by using SSV from the mongos with a csrs connection string as a signal to the shard to switch to CSRS mode.