Based on your description (including the indication that this is a one-time process), here is how I suspect I would do it:
Let’s first insert a column. I would probably insert before column A, but that would require us to re-identify the columns in your post – so for the sake of discussion, let’s insert our column as D.
Here is the formula for our new Column D (we’ll assume it is placed in cell D2): “=IF(A2=B2,C2,A2)”
- Copy this formula to the remaining cells in Column D corresponding to your dataset.
- Review the results in Column D.
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Copy Column D to Column A. Use "Paste Special" or "Paste Only" to paste only the results you want (i.e., NOT formulas). For instance, let's assume your "4-digit year plus the old number" results in a text string. You could "Paste Only > Text" to paste the string (not the formula) into Column A from Column D.
- Delete Column D.
You have now updated Column A with Column C, but only in cells where Column A and Column B are the same.
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