Answer
On Apps10, IFSSYS's password is changed from the IFS Middleware Server Admin Console rather than through the installer — log into the Admin Console, navigate to Common > Database, and click the 'Change password' icon. Two options are presented: one changes the password only in the Middleware Server's own configuration, while the other changes it in both the Middleware Server configuration and the actual database password simultaneously — normally you want the second option to keep both in sync. Changing this password triggers a restart of the IFS Middleware Server, making the application unavailable for a period, so this should be done during a planned maintenance window rather than during business hours.