Hi,
VPN 3000 Concentrator does not have option to automatically force internal users to change their passwords. User need to key-in their password manually from the VPN 3000 management GUI.
However, this can be done using Cisco Secure ACS, via User-Changable Password utility.
It will be good if you have ACS. You can download 90-days trial version for evaluation purposes.
http://www.cisco.com/kobayashi/sw-center/ciscosecure/cs-acs.shtml
The User-Changable Password config guide is available at:
http://www.cisco.com/en/US/products/sw/secursw/ps2086/products_configuration_example09186a0080094e7b.shtml
Hope this helps.
Rgds,
AK