When import constituent updates in a previous version the data entry settings for name updates would affect incoming records when identifying the constituents by their Lookup ID. In version 4.0 the constituent update rules no longer apply to records that are explicitly selected using a lookup field.
Download and install the latest service pack, which contains all fixes from previous service packs. If you are running an older version, download and install the latest version and then the service pack.
Steps to Duplicate
1. Go to Administration > Import 2. Run the import process 3. Open the resulting batch, the data in the last name field of the import file was updated. 4. Edit the import process 5. Select the Set Options tab > Constituent Updates 6. Notice that the option is set to Ignore the name if it's different