asset change but depreciation not change

In 2010.7.20, we change one asset useful lives from 3 years to 5 years, (straight line method) this asset last depreciation is on May.2009, monthly depreciation amount is 20K IDR, after that we didn't do any depreciation

now, in Aug 2010, we run depreciation for 2009.June, the planned depreciation is still 20K IDR, why system didn't calculate depreciation amount based on new depreciation useful lives? how can I adjust the system, so system will auto calculate the changed depreciation amount based on new useful lives?