Cont. to my previous comment,
To get the count of records already existing in DB table matching to records in internal table, you can use following statement:
SELECT COUNT( * ) INTO L_COUNT FROM DB_TAB
FOR ALL ENTRIES IN INT_TAB
WHERE KFIELD_1 = INT_TAB-FLD1 AND KFIELD_2 = INT_TAB-FLD2.
IF SY-SUBRC = 0.
" you will get no.of records existing already in L_COUNT.
ENDIF.
Then find the Total No.of records in your internal table (Describe Table) into L_LINES.
Modified_records = L_LINES - L_COUNT.
Regards,
Vijay