Currently if I select a new data source for a mail merge document, I have to manually put in new fields to pick up the information from the new data source. I have no idea why someone would think this is a good idea but it definitely isn’t.
So long as the new data source contains the same fields as the one one the document was created against, it should use the fields from the new data source.
The current behaviour is especially painful when the field contain conditional text or formatting as the coding for that also has to be redone. It almost feels like the developers don’t want people to use the mail merge feature.