First of all, in order to customize the new form, you need qListForm web part. If you have qListForm web part, I think you can do what you would like to do. If you configure the Display Fields property of the qListForm webpart, you can configure different set of fields for different content type. In the discussion list, the top level discussion is represented by Discussion content type. The response is represented by Message content type. So, you can set different fields for each content type. I hope this helps.
Development Manager – Quest Web Parts for SharePoint