Hi Nachiket,
In EL29 , Meter Reading Date & Meter Read status cannot be changed, Its in display mode.
In EL27, Meter Reading Date can be changed but not the Meter Read Status(display mode).
If you are seeing changes to date and status after billing , this is definitely cause of custom developments. Please check the code.
If you are on EHP6+ system, you can check table EABLDEL if the meter read document is deleted.
Hope this helps. 
Cheers,
Rakesh..
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