How to change default accounts set for an average costing inventory organization?

Our customer is using average costing method and would like to change the default accounts set up for a transacting organization.  However, these fields have been greyed out by the system, not allowing update to its initial values. 

1.  Outside Processing

2.  Material Overhead

3.  Overhead

4.  Resource

5.  Expense

6.  Cost Variance Account

I think it's kind of overkill if I will opt to use a customized ADR and change the Cost Management SLA just to correct the values.  Furthermore, the customer doesn't want to create a new inventory organization to preserve the transaction trail and history.

Thanks.