Depreciation method helps accountants in aligning the recognition of cost with the benefits rendered by asset as per the requirement of IAS 16 that requires rate of depreciation rate to match the rate at which benefits are extracted from the asset. Thus depreciation method in itself is an estimation of consumption of utility in the asset.
On the same footings, change in depreciation method is not a change in accounting policy rather it is a change in accounting estimate. Change in accounting policy only occurs if rules of either recognition, measurement or presentation of line item are changed. Change in depreciation method changes neither of these. Therefore, it is a change in accounting estimate.
This is further confirmed by IAS 8’s definition of change in accounting estimate
A change in accounting estimate is an adjustment of the carrying amount of an asset or a liability, or the amount of the periodic consumption of an asset, that results from the assessment of the present status of, and expected future benefits and obligations associated with, assets and liabilities. Changes in accounting estimates result from new information or new developments and, accordingly, are not corrections of errors
Depreciation is an adjustment of carrying amount of asset over its useful life to correctly project the state of affairs of business. And on receipt of information if it is found that previous estimations were wrong regarding devaluation of asset then change in depreciation method is brought.
Additionally, change in scrap value of asset as a result of new developments will also trigger adjustment in accounting for depreciation and it is also a change in accounting estimate and not a change in accounting policy.
A change in accounting policy in relation to depreciation charge will occur if entity changes its policy whether to depreciate the asset or not. Change from historical cost basis to revaluation basis is a change in accounting policy. Changing the way depreciation expense is presented in the financial statement for example previously a depreciation of certain was recognized as part of admin expense but now it is included in cost of sales then such change is a change in accounting policy.
Accounting for change in estimate, it is treated prospectively whereas change in accounting policy is brought in a retrospective manner. To learn how change in depreciation method is accounted for read: How can we change the Depreciation method?
Thanks to Mezbah Uddin Ahmed for pointing out the matter as it might cause confusion.