Per this document, you need to set the client to Client Control mode before making it unmanaged:

https://www.symantec.com/business/support/index?pa...

If the communication mode was not set for Client Control in the Symantec Endpoint Protection Manager policies during initial installation, you will not be able to change the local client policies after placing the new Sylink.xml file on the client.

To set the client to Client Control

• Log in to the SEPM
• Click on the Clients page
• Select the group the client is in you want to make unmanaged
• Click the Policies tab
• Under Location-specific Policies and Settings, click the + next to Location-specific Settings
• Click Tasks next to Client User Interface Control Settings and select Edit Settings
• Select the option for Client Control
• Click OK

Make sure the policy update takes affect on the client. Once it does, you can than make the client unmanaged and you should than be able to change all the settings.