I'm trying out incubator/mongodb-replicaset. It works quite nicely so far. I installed it successfully. I then made changes to the ConfigMap containing the mongod.conf and tried to upgrade the chart. The result was this:

Error: UPGRADE FAILED: StatefulSet.apps "mongo-mongodb-replicaset" is invalid: spec: Forbidden: updates to statefulset spec for fields other than 'replicas' are forbidden.

The only thing that I changed is the ConfigMap. The docs state that Helm only updates things that have changed since the last release. The ConfigMap was actually updated correctly even though the above error was the only output.