>>>Anyone have any idea why sometimes the null fields come across as null and sometimes they don't? Anyone have any idea how I can make sure they come across as null? Thanks!
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>>What if you change the update type to Key Field Only?
>
>I thought about that but maybe I don't understand it... this is not a key field (i.e., not used for indexing or anything -- is that the correct understanding of the term?). So if I update key field only, wouldn't it prevent the updating of any field except for my one key? Maybe I'm not understanding?

No, it means only the fields in the key field list will be used to compare to the source data to determine if there is an update conflict. All changed/new data will be updated.