Change of asset base value for depreciation

Hi,

My client is currently depreciating an asset @8.33% per annum on SLM Basis. From 01.04.2010. Depreciation rate to be changed to 5.38% and the system should calculate the depreciation on WDV / Book value of the Asset as on 31.03.2010.

If I create new depreciation key with 5.38% and assign to the respective asset, then the system will calculate the depreciation on Asset procurement value.

How to change the system to calculate the depreciation on WDV value on 31.03.2010.

Help me with this please.