Well, when the servers came back up you'd have access to your archived emails, right?
Doesn't Yahoo use webmail?
Could you display an email, then do a "save as" and save the file to a foder on your HD? Would you have to do that for each individual email as a separate file... that is, not easily download your email folder and save them all at once ?
In Outlook I use File --> Import/ Export to move contacts and e-mail from one system to another. Works very well. I choose the Windows CSV (comma separated values) format to get it to export to Excel. Works very well.