Hi,
Thank you for posting in Symantec community.
I would be glad to answer your query.
You were under wrong assumption. This is how it work, mentioned below.
When your current license is about to expire, the Symantec Endpoint Protection Manager (SEPM) begins sending license expiration notifications to the Symantec Endpoint Protection administrator. Symantec highly recommends that you renew your license before it expires.
When you renew a license, the expired license is removed and is replaced with a new license. To purchase renewal licenses.
Reference: About renewing your Symantec Endpoint Protection license
http://www.symantec.com/docs/HOWTO55071