Changing Asset class from Depreciation key X400 to X250

Hi SAP Gurus

I want to change an Asset class i.e. M02 from Depreciation key X400 to X250. I can do that by T - code OAYZ. Issue goes beyond that and client's requirement is that Assets acquired after 02 May 2008, depreciation key should default to X250 and Assets acquired before 02 May 2008 should default to Dep. Key X400. Another dimension is that Assets acquired before 02 May 2008 but closed out after 02 May 2008 should also default to X400.How is that possible.

Regards

Satish Kumar