Copying Data without overwriting (edit macro)

Let me explain a bit detail how the macro works.
- We compare every value in sheet1 column C with sheet2 coloumn A to find a match

If there is a match then do this in that respective row of the matching value in sheet2:
- copy the data from the respective columns in sheet1 (D,F,H) and paste it in the sheet2 columns F, D, I respectively.

Now what I want is that when the value are to be pasted in the columns of sheet2 they should not overwrite the already entered values in the respective cell. it should just move on.

I hope this clears your question.