Bulk E-mail Forwarding?

OK. I just came up with an idea, but I don't know how to implement it. I have a "boatload" of email in one account. I would like to move it all to a different email account, but I don't want to open each message and forward it to the new address. Is there a way to bulk forward email to another address? How could this work with web-based email such as Hotmail and Yahoo?

I appreciate your thoughts. Please add a guidepost. Thanks much!