check data in column D and replace with updated data

I have the attached workbook that I need code to check if the new data contained in sheet2 "Modified" is different in column D compared to the original data in sheet1 "Original" and replace it with the updated value in column D. The key fields are column A and column C. I cannot just delete the original data as it is referenced elsewhere so it just needs to be column D.
Thanks in advance.