Is the change in depreciation method i.e. from straight line to reducing balance is change in estimate or change in accounting policy?

as per IAS -8 its a change in accounting estemate . 

It is as an acconting estimate as 

• A change in the measurement basis applied is a change in an accounting policy,and is not a change in an accounting estimate.
• no retrospective effect is carried out in case of change in  depreciation method.

Changes in estimation technique

FRS18 also covers changes in estimation technique. Any changes to estimation technique are not accounted for in the same way as changes in accounting policy. Instead, any changes in estimation technique are accounted for prospectively