Ability to maintain default expense account based on organization cost segments?

Is there a way to trigger an update of the default expense account based on the Organization populated on the assignment?  We are using the default expense account on the assignment's purchase order tab for iExpense.  Whenever HR makes changes to the employee's organization, we would like the expense account segments to update based on the new organization's costing segments.  We cannot rely on HR staff to remember to update this as it is not used on all of the employees.  We thought we could use Forms Personalization to do this, but not sure how to handle date tracking.   Has