WordPress.org

Does Extended Comment Options hide existing comments?

No, but the theme you're using may hide comments on posts if comments are closed. If you're comfortable with PHP you can change the theme yourself in the single.php file in your theme folder. If you're not, you can contact the author of your theme or try the support forums if you need help.

What about WordPress version 1.5?

There is a version of Extended Comment Options available for WordPress version 1.5 from the Plugin Homepage.