A common method to keep all the posts open for comments, is to add this line into the file functions.php
in the current theme directory :
add_filter('comments_open', '__return_true');
where __return_true
is just a core function that always returns true
.
ps: since this seems to be helpful, regarding to the problem described by @user27309, it’s better to have the comment solution, added as an answer 😉