FeatureWriter Updating Esri FileGeoGDB (ArcObjects)

I try to update FeatureClasses in an Esri FileGeoDB.

I use FeatureWriter with fme_db_operation=update and dynamic schema setting. FeatureClasses have NOT NULL attribute constraints.

Problem: How do I control which attributes are updated ?

I have the impression, that I cannot update only some attributes without specifying all attributes of the FeatureClass/Table. If I do not supply an attribute it will be set to NULL.

Is that really so ? Is there a way to leave attributes unchanged, while others are updated ?

That's what I am used to from SQL ...

Thanks

Michael