How to change "add to comment" in comment form?
Create a seperate .tpl file for comment-wrapper named comment-wrapper.tpl.php. 

To create a seperate template file for node specific comment wrapper you can create using
comment-wrapper-contenttype.tpl.php copy the code from core comment-wrapper.tpl.php and change the title in the code given below.

While copying from core comment-wrapper.tpl.php don't forget to remove the validation which is meant for forum node types only.

&lt;h2 class="title comment-form"&gt;&lt;?php print t('Add new comment'); ?&gt;&lt;/h2&gt;