Downward revaluation lecture question doubt

Dear tutor
This doubt is related to the question in the lecture itself
when we revalue upwards we transfer the accumulated depreciation to the revaluation account to bring The Asset a/c back to its historical cost and then adjust it through the revaluation account so when we revalue downward in the lecture why don’t ‘t we transfer the accumulated depreciation of 1750 to the revaluation a/c if we do so then we can directly Cr 6000 in the asset a/c and show the balance as 8000 (14000-6000). it’s like I don’t really understand because every time I do upward revaluation I always transfer the depreciation to revaluation a/c and adjust the cost could you please explain where I’m going wrong, do note that I try to solve ppe adjustments through T accounts instead of statements also can u kindly mention what impairment means.
and in the case of impairment in oci of the $3850 since it is already adjusted From the revaluation a/c and the asset a/c why are we mentioning it again in the oci since there is no double entry for it