Hi Allyson,
The yellow tabs (SourceSchema, TargetSchema, SourceSubtypes, TargetSubtypes) in the workbooks are for your reference only - for you to look at and help you better understand the source and target schemas of your data models. Also, the source field options you get in the dropdown (Expression column, in the Mappin tab) come from the SourceSchema sheet - so if you remove a field from that sheet, it will not be reflected in the dropdown anymore, but it can still be used to load data. Changes you make in these yellow tabs will not affect/impact the results of your data load.
If you make changes to your source or target datasets, you will need to make the related changes in the Mapping tab and the blue tabs to reflect these changes.
So to answer your question "If a field has been added to the UN data model and that field has values in the source data, it does NOT need to be mapped in the mapping tab? even just !Latitude! to the new latitude field?" - Yes you can add that new field in the mapping tab, in the TargetField column, and assign the source field in the Expression column.
When making changes to your data models after having already generated the workbooks, the recommended approach is to run the Create Data Loading Workspace tool again, against your modified source and target datasets, and create a new separate Data Loading Workspace that you can use to compare the changes and implement them in your workbooks.