Create a catalog (or in XP and later it's called directory) type mailmerge
main document that uses the same datasource and in which you have a one row
table with the mergefields in the cells of that table.  When you execute the
merge to a new document, it will contain a row of data for each record in
the data source.  If you then insert a row at the top of the table and enter
the names of the mergefields into that row, you can use that document as a
datasource if that is the easiest place to make any revisions.