Hi Donna,
We did exactly this at the Shetland Archives a few years ago. CSVs exported from CALM then cleaned up manually using LibreOffice Calc was the method we used. We did it fond by fond, starting with the smallest ones first until we got better at it. The work was painstaking, and slow - there were about 100,000 CALM records and, working on it pretty much all the time, the export took me about a year and a half. It was 100% worth it, though. AtoM is far superior to CALM, in the opinion of the three archive staff who work here, and, after months of going through spreadsheets I now know the catalogue far better than I did before.
Mark