Its not possible to change the encryption password.
https://www-secure.symantec.com/connect/forums/sepm-installation-question-about-password
The Encryption Password and Symantec Endpoint Protection 11 (SEP11)
http://service1.symantec.com/support/ent-security.nsf/854fa02b4f5013678825731a007d06af/1af09196831f2c208825758500670bf8?OpenDocument
If, during the configuration portion of the SEPM, the "Simple" configuration option is chosen, then the encryption password should be the same as the admin login for the console (see page 66 of the install guide). Please note, however, that resetting the admin login does not reset the encryption password.
If an "Advanced" configuration was performed and a different passphrase was entered, there is currently no way to recover or reset the encryption password. The only resolution, at this time, is to reinstall the management server with a new encryption password (place this password in a very secure place, ie., a safe) and push the new sylink.xml file to all of the agents so that they get the new SEPM GUID.