I have done it in my site. Thanks to Rafael's guide(and fix the bug).
Here's the steps:
1.edit the file templates/default/post_form.htm, find the following:
change it into
2.add a new key "PostForm.replyTopic" in jforum_xx_XX.properties
3.restart
Tomcat
BTW. If you want to do more, you can add a PrivateMessage.replyTopic for distinguish starting or replying a Private Message and follow the same step to finish it.
[originally posted on jforum.net by andowson]