We are planning to use Caché users on a SOAP web-service, so the WS-security tokens will be used.
It will be username and password only for now.
The passwords should expire on a regular basis and this will be configured in the system-wide security settings.
The consumer of the web-service should be able to change their password on-demand or when it has expired, via a web-service call.

For the on-demand change, I can create a service method which can be called by the consumer to change the password.