Importing Old E-mail to Yahoo AT&T Portal

I'm seriously considering uninstalling T'bird, and exporting all of my old e-mail folders to the AT&T/Yahoo.com webmail portal. It's not that T'bird doesn't work, it works fine. I just thought it might be a little more secure, and clear some of the 2Gigs of clutter on my hard drive that gets fragmented pretty quickly. Also, if my hard drive ever crashes unexpectedly, I won't have to worry about that aspect, which is the only dynamically changing component on my C: drive.

So, my question is, as of now, I'm not aware of any shortcut, efficient way of exporting/importing my local folders to my webmail portal. Are there any brainiacs around here that know something I don't know?

Thanks,
GPL