I have a comcast.net email account, and I recently changed services. I want the people that send stuff to my comcast account to either have it forwarded to my new yahoo account, or at least get a message that says my email address has changed. Any idea on how I would do this without obviously emailing everyone and telling them that it has changed?

call your old ISP, they can hook you up

There is always an option on your ISP and in case of CONCAST i know for sure that you can setup a automatic reply to every message that hits your inbox, so in that message you say that your e-mail is changing to yahoo and vuala.

Normally it is called " automatic reply" but it might change from ISP to ISP.