Change in the life of the asset and the depreciation posting of the same

Dear Experts,

We require to change the life of an existing asset from existing 15 years to 25 years. This we are able to do.

The problem is for posting of depreciation.

We want the system to post the depreciation on the Net Book value (On the date of change of the life of the asset) instead of the original value. for the remaining life of the asset.

Depriciation for this Asset has been posted upto May 2011.

Plz explain on how this can be done in SAP.

Thanks