/feed/ to .xml mod rewrite

Here’s the situation. A few months ago I started a PodCast and delivered MP3s through my RSS feed. Now 1100+ downloads strong, my show is booming and I moved to wordpress! Unfortunately I was using MovableType at the time, so my subscribers all have my old address

http://tracks.upthetree.com/index.xml

I am trying to, unsuccessfully so far, invisibly substitute my new RSS content for the old .xml file without a change in the URL

New feed is http://www.upthetree.com/category/tracks/feed

Can it be done? Thanks in advance.