You were close. In your advanced find on the entity, include the new fields (even though they are currently empty). Export that to excel.
Do not alter columns A, B, or C at all. Now you can update the empty columns for the new fields with data and import the file this way. This will update those fields.
We use this approach to update hundreds of thousands of records in our implementation.
Vice President, FIS Sales Tools, Infrastructure
4900 West Brown Deer Road, Brown Deer, WI 53223
FIS | Empowering the Financial World image005.png@01D11265.2C7F1D60">image006.png@01D11265.2C7F1D60">image007.png@01D11265.2C7F1D60">