Organization replacements- Things to be considered

Hi,

We have the below requirement:

Have to change costing method from standard to average. Already items and transactions are exists.
I hope we don't have any other option other than creating new Inventory organizations.
In order to this change, what are all the standard set ups to be considered/taken.
Ofcourse we will be analying all the custom components.
Apprecitae your help.

Thanks.