The differences between 12.1.x and 14 are outlined in this Symantec KB article:
http://www.symantec.com/docs/HOWTO125381
Once you're on 14, upgrades to future versions of 14 will be smaller.
14 also uses the Intelligent Threat Cloud Service which minimizes the virus defs used for the standard client. It now uses content in the cloud.
A reboot is needed to complete the upgrade from 12.1.x to 14.