comparsion of two records

Hi,I have two files, A and B. The records in both files share the same format and the first n characters of a record is its unique identifier. The record is of fixed length format and consists of m fields (field1, field2, field3, ...fieldm). File B contains new records and records in file A that have changed. How can I use cloverETL to determine which fields have changed in a record that appears in both files?