changing standard cost to average cost in 11.5.10.2

Hello,
i am working on a client who needs to change the standard costing method in multiple warehouses to average costing. What is the best approach to follow. I am looking for some ideas other then creating a new warehouse with average costing as its costing method.

Also what are the disadvantages of forcing a change of the costing method to average costing? Will there be any issues in historical data such as SO, PO, WO?

Appreciate any input on that

Thanks,

Philip