Is there a way to change the datasource without word losing the field
matching?

I was trying to set up a basic template that someone else could load,
select a file that is emailed to them and then hit merge to printer.
The problem is when the file source is changed word remaps all the
field names (incorrectly) instead of keeping the current mappings. The
file itself will always contain the same headers in it as it comes from
an automated export from a database that is then emailed to us.