Updating actual and remaining work from Excel not updating properly

I am using MS Project 2007, SP2, and I'm trying to update actual and remaining work on assignments through an excel 2007 import (using xls format). I started by exporting the assignments from the project into excel, with the Assignment Unique ID to be used as the MergeKey, and then updating the resulting excel spreadsheet with new info to be brought back into Project.

When I do the merge (not append), the actual work and remaining work do not update. If I do an append, new assignments actually do get created.

The fields that I am using for the update are:
Unique_ID (which is the assignment ID)
Task_Unique_ID
Resource_Unique_ID
Resource_Name
Actual_Work
Remaining_Work
Work

I created a simple 15 line plan to try to get this to work, and would appreciate any help.

Try the export/import without the work field -- use only Actual and Remaining Work and see if that helps.

Julie