Change in deprection key

Hello SAP expert,

Hope you can help, Currently in SAP 4.7 Enterprise I am trying to create a depreciation key that will do the following.

Currently we have assets that have been depreciating straightline over 4 yrs to 15% residual value.

This month (period 10) we want to change the depreciation for existing assets to start depreciating from net book value to zero for the remainder of usefull life.

I have managed this except the first period (10) is calculating a catchup amount, what do I have to do so that period 10 and subsequent periods are smoothed out without the catchup.

ie Netbook value (period 9)/remainder of usefull life.

Regards Steve Dunford