Hi Ashish,
It is funny that auditors asked to change the signed Financial statements. No accounting standards requires this kind of change. What you could have done in 2013 is to show the balance depreciation as prior period items. But you need not open the books of 2012 & 2013 now. But you need to modify certain depreciation key configurations so that depreciation expires within the usful life.
In T code AFAMS give the Multi level method used in Depreciation key (T code AFAMA) and change the base value as "24-Net book value" and tick the remaining useful life. This settings ensures depreciation expiry and useful life end coincides and no depreciation will be missed out.
Hope it is clear.
Regards,
GSR