If useful life of computer software is revised from 5 years to 2 years, this would be a change in estimate as per IAS-8 and change will be prospectively.
However, in practical example
Cost $ 50000 as on 31-12-09
Accumulated Depreciation $ 30000 as on 31-12-09
NBV 20000 as on 31-12-09
If life is revised from 1-1-10 from 5 years to 3 years then
NBV 20000 would be amortized over 2 years from 1-1-10.
but what if some softwares have already finished their original life after 3 years, what would be effect.