"Rewritten data is different from original Hex Code data. "

What do you mean "different" ???

is the data:

unchanged ?
000's
111's
corrupted ? (resembles data, but not perfect copy)
garbage ? (no pattern detected)

Do you write to 29c256 using code running on rom emulator ?

It is not possible to execute code from the 29c256 (or any other flash of that family) AND write to it, because it becomes busy when you write to it. So please explain how you are writing to it.