Kind of a hack solution, but try running this script:

#!c:\perl\bin\perl.exe

use strict;
use warnings FATAL => 'all';
use Fcntl qw[:flock];

my $FH = 'C:\\scripts\\20040120\\mr\\imp\\';
chdir $FH or die "$!";

while (glob '*Jan2004*.txt') {
    open FH, $_ or die $!;
    flock FH, LOCK_SH or die $!;
    while (<FH> ) {s/n\/a//}
    close FH or die $!;
}

print 'replacing completed'
[download]

untested, but theoretically, that oughta work :)