linux$ cat t.c
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/mman.h>

int main(int ac, char *av[])
{
  struct stat st;
  char *m, *e;
  int fd;

  fd = open(av[1], O_RDWR); 
  if (fstat(fd, &st) < 0) { perror("fstat"); exit(1); }
  m = mmap(0, st.st_size, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
  if (m == 0 || m == (void *)-1) { perror("mmap"); exit(1); }
  e = m + st.st_size;

  while (m < e) {
    if ((m = strchr(m, '\n')) == 0) break;
    if (*++m == 'T') break; 
    if (*m != 'D') m[-1] = ' ';
  }

  return 0;
}

linux$ gcc t.c

linux$ cat t.dat
HEADER474687
D1356jkl ugbliuybikb 879870
898976098 9687680
D77656757 uhgliug liygoiygig
D98679hjh kjbgihguygfu ugliyh
kbygfluy9809
D8796870 kjlhuigiyig
TRAILER0008

linux$ cp t.dat t1.dat

linux$ ./a.out t1.dat

linux$ cat t1.dat
HEADER474687
D1356jkl ugbliuybikb 879870 898976098 9687680
D77656757 uhgliug liygoiygig
D98679hjh kjbgihguygfu ugliyh kbygfluy9809
D8796870 kjlhuigiyig
TRAILER0008

linux$ cmp -l t.dat t1.dat
 41  12  40
118  12  40