depreciation valuation method from WDV to SLM

Dear SAP guru's,

My client requirement is need to change depreciation valuation method from WDV to SLM.

Form past 3 years client calculated the Depreciation on WDV method now company has decided to change to SLM and re valuate the depreciation for all the assets.

a) How to change the valuation method we just need to change depreciation keys or we require additional configuration for this purpose.

b) After re valuating the depreciation Client need FI entry should be

Accumulated depreciation account Dr

Depreciation account Cr

Thanks in Advance

Rita