Accounting Policies & estimates

Hi,
Change in depreciation model from cost to revaluation is a change in accounting estimate or policy?

“If a non-current asset has a depreciable amount of $5,000 to be written off over five years, different depreciation methods such as straight line, reducing balance etc. all represent different estimation techniques. The choice of method of depreciation would be the estimation technique whereas the policy of writing off the depreciable amount non-current assets over their useful lives would be the accounting policy.
Estimation techniques therefore implement the measurement aspects of accounting policies.”

The above para has been taken from the Kaplan study text. What i understand from this is that if a company first shows an asset at cost(purchase price) and all its depreciation would go to an accumulated depreciation account and later if the company starts writing off the depreciation from the cost (thereby eliminating its accumulated depreciation account) that would then be a change in accounting policy.

Please correct me if I am wrong and if you could then explain what they’re trying to say that would be very helpful.
Thanks in advance!