i'm still not sure what exactly you are trying to accomplish. if you are just downloading tables from source to destination, and need to replace type with documentno if type="credit", you could use sql statement in datasource, something like
WHEN TYPE = 'credit' THEN DOCUMENTNO
END AS TYPE
If you are just trying to update destination table with source data, you could use sql statement in source again, select primary key and document no
SELECT [lineno], documentno, type
WHERE (type = 'credit')
(I assume that lineno is your primary key, and then use ole-db command to update the table with
update destination set documenttype=? where [lineno]=?