Hi,
Thank you for posting in Symantec community.
I would be glad to answer your query.
For new users, You can install unmanaged Symantec Endpoint Protection client software by using the Setup.exe file on the product disc. The installation program installs the client software by using an installation wizard.
OR
You can create unmanaged client package from the SEPM as well.
Check this article: Create an unmanaged client install package from the SEPM with custom policies.
http://www.symantec.com/docs/TECH105498
Now, for existing users either replace Sylink.xml or uninstall existing version and install unmanaged client.
Replacing Sylink.xml can be easier way. Take the unmanaged client Sylink,xml and replace it on managed client. Need to stop the service prior to replace sylink.xml.