Useful life change

1: At year end

Depreciation =
Carrying amount / (Useful life + 1)

This is according to John Moffat.

Is this because IAS 8 requires Prospective application on Current and Future periods?
Does it mean that even if revision is done at year end and on Current period, all the past the past months of such a period are treated as prospective?
I am insisting because that is how Sir Moffat explains it.

2:At start of period

Depreciation =
Carrying amount / Useful life