My main Mu blog is johnmunoz.com
my old domain name was johnmunoz.net with my old feed at johnmunoz.net/feed
I've created an alias for johnmunoz.net pointing to johnmunoz.com. When users visit my old feed, they just get redirected to the new main page.
My sub sites work fine.. but the main site doesn't give me the option to map the old domain name to the new site.