How to depreciate existing assets with useful life

Hi

Presently our all Assets are Depreciated on percentage Basis. Now Management wants to depreciate Existing Assets with useful life.

For Example: Original Cost  = 10000

Depreciation Rate = 10%

Date in Service = 01-APR-2012

as on 31-Mar-2014 Net Book Value = 8000

Now w.e.f. 01-o4-2014 we want to change the Depreciation Method & depreciate Asset with Useful Life i.e. 2Yrs . Depreciation Should be calculated on balancing amount i.e. Net Book Value  

See Below calculation 

8000.00 -  Net Book Value

4000.00 - Depreciation per year (333.33 per month)

Please advice