On the Management Console select Products. Scroll to the bottom of the page and on the left hand side click on the arrow on the Import/Export tab. From there, select Products again to arrive at the Import/Export page.
Select "Advanced" and click into the "Additional Fields" box. Select "Reset SKU" and/or "Reset Barcode" depending on what is to be replaced. These fields will create additional columns in the product export that allow for new SKUs/Barcodes to be added.
Click "Export" to download the Excel file. Input the new SKUs or Barcodes in the appropriate column, save and re-import via the Import/Export page.