If you previously used an imap account and the new yahoo account is also Imap then all emails should be available on server. But you might need to subscribe to see any folders you created. If you previously used a pop account and are now using a yahoo imap account then it is possible to 'Copy' batches of emails in a Pop account folder to an imap account folder to upload them onto the yahoo server. Alternatively, copy pop account emails into 'Local Folder' mail account so they are still visible when you finally remove the Pop account. This means you can still see emails but you not be using up any additional server quota.