I am not familiar with the option to change password on Radius itself. I thought it was in conjuntion with a NT Domain server. In any case, there are two type of radius setting on the concentrator, one is plain radius, the other is radius with expiry, make sure you select radius with expiry.
Otherwise, here is a link for the config guide on NT Domain password expiry :
http://www.cisco.com/warp/customer/471/vpn3k-ntpwexp.html .
Regards,