I agree in principal to this thread. I think that if you are going to change the sales letter, then you should check with the original author. In my experience, most don't really mind anyway, as their income comes from within the ebook, not the sales letter.
If you can sell more copies, they make more money.
I recently read Ewen Chia's Resale Rights Secrets (http://www.resellebooks.biz
) and in there he recommends completely re-designing the sales letter! His view is that if you are one of 1,000 or more people selling the same title, you need to differentiate yourself from everyone else.
Just my 2 cents worth.