Depreciation (2)

(BPP)
Q)
At 31 December 20X3 Q, a limited liability company, owned a building that had cost $800,000 on
1 January 20W4.
It was being depreciated at 2% per year.
On 31 December 20X3 a revaluation to $1,000,000 was recognised. At this date the building had a
remaining useful life of 40 years.
What is the balance on the revaluation surplus at 31 December 20X3 and the depreciation charge in the
statement of profit or loss for the year ended 31 December 20X4?
Depreciation charge for Revaluation surplus
year ended 31 December 20X4 as at 31 December 20X3
(statement of profit or loss) (statement of financial position)
$ $
A 25,000 200,000
B 25,000 360,000
C 20,000 200,000
D 20,000 360,000

ANS:
B Revaluation surplus – (1,000,000 – (800,000 – (800,000 ? 2% ? 10))) = $360,000
Depreciation charge – (1,000,000/40) = $25,000

My doubts
1) Until 20W4 the asset was depreciate at 2% per year. After that the asset value is divided by the number of useful years left and depreciation is calculated. This interchange has happened quite a lot of times. How to know when to interchange and when not to?

Please excuse me if my doubt is silly.
Thank You