On clean startup of a mongod with the --shardsvr option, the FCV is defaulted to the “last-lts” to ensure compatibility when being added to a downgraded version cluster. The config server will run setFeatureCompatibilityVersion on the new shard as part of the addShard command.

In order to support the ability to add a new shard into a cluster that is running the “last-continuous” version, we must allow the new shard to be able to transition from the “last-lts” to “last-continuous” FCV. We will add a new optional fromConfigServer parameter to the setFeatureCompatibilityVersion command that will be called by the config server as part of addShard. Invoking setFeatureCompatibilityVersion with fromConfigServer: true will allow a shard server to transition from “last-lts” FCV to “last-continuous” FCV.