No. You can upgrade single KM also.
In BMC Helix Operations Management, to use the new configurations added to the policy in the upgraded KM, you will clone the policy and create a new one by using the cloned policy.
In TrueSight Operations Management, you can change the KM version in an existing policy and configure the new configurations added in the upgraded KM.
In both the cases, you can also leave the existing policies as is and create a new policy to utilize the new features.
You will continue to see the collected data under the device that you configured for collection. If you ensure similar device configurations in the new policies, the new collected data will also show under the same device.
You do not need to update the exisitng thresholds. However, set up thresholds for new attributes added to a release.
You do not need to set up event policies for existing attributes. However, set up new event policies or update the existing ones for new attributes added to a release.