The following Source fields are updated by gifts being posted to those Sources. If you are not uploading any of these in the file, then there should not be an issue:
If the Mail_Date on the Source record in TA is Null, then you can update any field on the Source record with uploaded data (though you do not want to upload information in the above fields if gifts have already been posted to the sources). However, if there is a Mail_Date on the Source record in TA, then when you do a source upload these are the ONLY fields that it will update:
Note: the Number of Solicitations on a Source is not updated if a Mail_Date exists on the Source record.