I want to alter the categories on all of my items. If I export them to a CSV file, then alter the fields I need changed, and then import back it would make the process much easier. This way people could contiune to work as I make the changes, and I won't have to open each item.

My question is: Can the import function be used to edit items? or is it simply for adding items?

If it can be used to edit, can I just import the fields that I want changed, or will it blank out fields that there is no import data for?