;;; foo.el begins here

;; input.txt :

;; mmmm;NEW;24
;; nnnn;OLD;25
;; nnnn;NEW;26
;; no pair oooo;NEW;28
;; pppp;OLD;29
;; pppp;NEW;30
;; no pair qqqq;NEW;32
;; rrrr;OLD;33
;; rrrr;NEW;34
;; ssss;OLD;35
;; ssss;NEW;36
;; aaaa;OLD;1
;; aaaa;NEW;2
;; bbbb;OLD;3
;; bbbb;NEW;4
;; cccc;OLD;5
;; cccc;NEW;6
;; dddd;OLD;7
;; dddd;NEW;8
;; ffff;OLD;9
;; ffff;NEW;10
;; no pair gggg;OLD;11
;; hhhh;OLD;13
;; hhhh;NEW;14
;; iiii;OLD;15
;; iiii;NEW;16
;; jjjj;OLD;17
;; jjjj;NEW;18
;; kkkk;OLD;19
;; kkkk;NEW;20
;; no pair llll;OLD;21
;; no pair mmmm;OLD;23
;; tttt;OLD;37
;; tttt;NEW;38

;; run:
;; $ emacs -batch -l ./foo.el -eval '(foo "./input.txt" "./output.txt")'

;; output.txt :

;; 25 = 26
;; 29 = 30
;; 33 = 34
;; 35 = 36
;; 1 = 2
;; 3 = 4
;; 5 = 6
;; 7 = 8
;; 9 = 10
;; 13 = 14
;; 15 = 16
;; 17 = 18
;; 19 = 20
;; 37 = 38

(defun foo (src trg)
  ;(interactive "fSource file :")
  (let ((result ""))
  (with-temp-buffer
    (insert-file-contents src)
    (beginning-of-buffer)
    (let (line text type id p)
      (while (not (eobp))
        (setq line (thing-at-point 'line))
        (if (string-match "\\([a-zA-Z ]+\\);\\(OLD\\|NEW\\);\\([0-9]+\\)" line)
            (progn
              (setq text (match-string 1 line)
                    type (match-string 2 line)
                    id (match-string 3 line)
                    pair nil)
              (cond
               ((string= type "OLD")
                (setq pair "NEW"))
               ((string= type "NEW")
                (setq pair "OLD")))
              (progn
                (next-line)
                (let ((nline (thing-at-point 'line))
                      (nid nil))
                  (if (string-match (format "%s;%s;\\([0-9]+\\)" text pair) nline)
                      (progn
                        (setq nid (match-string 1 nline))
                        (setq result (concat result (format "%s = %s\n" id nid))))
                    (previous-line))))))
        (next-line))))
  (with-temp-file trg
    (insert result))))

;; foo.el ends here