Change reply-title from H3 to H6 (using comment_form) (Without Plugins)

hi..
in my blog, in comments section, i am calling comment_form() to display the input fields.
Hence, i cannot edit the code directly and possibly need to write a function() in functions.php.

looking at the code, it is a H3 wrapped in DIV:

<div id="respond">
<h3 id="reply-title">............
.................................

now, i would like to own H3 for my posts itself, and make that reply-title a H5/H6.
Anybody knows how to make the changes?

If the above is absolutely not possible, whats the perfect code replacement for comment_form() so i can rewrite and redesign the whole thing?

Using WP 3.5.
Please dont suggest plugins. Thanks.