I would not make any change to farm level properties like that. It is not something that the users will ever see and there is a chance that there is configuration data that is tied to that name.
Things I normally do to make it clear which environment you are in include:
- Set a good FQDN like portaldev.acme.com, portalqa.acme.com, portal.acme.com
- Change the Site Name to include the environment for Dev/Test/QA/Staging, etc
- Change the theme for the non-production sites so that it is clear which environment you are in
- Add a feature that can be enabled to add a user control to the masterpage that reads the FQDN and conditionally displays a note for non-production environments
Update: The SPFarm.DisplayName property supports only a get, not a set and cannot be changed. The SPFarm.Name property supports both get and set so it could conceivably be changed.