The documentation for "Create a Group" says this under the "Create a Group" heading:

Choose a globally unique name for your group; this name must be unique within the Ops Manager platform. For security and auditing reasons, you cannot use a name used earlier, *and once you name a group, the group’s name cannot be changed.*

(emphasis added)
As of Ops Manager 3.4 this is no longer accurate. The documentation should be updated to reflect the ability to change group names in Ops Manager as of version 3.4.

https://docs.opsmanager.mongodb.com/current/tutorial/create-group/#create-group