Importing Log Data

I want to reorganize all the log entries in a schedule. The plan was to export via dbf to xls, manipulate the data and then reimport to P3. All goes well until I find that an empty imported log field won’t replace a log field that already contains text. Trouble is Global Calcs don’t seem to handle log-data, or else I would clear all the fields that way and then import.
Any Ideas?
Sean