New Hard Drive, Mail Problems

I just had a new hard drive installed and backed up from my Time Machine. Now Mail, instead of just opening and importing new massages from my Yahoo Small Business email account, Mail says it needs to import messages in order to open. Does that mean all the messages and files that were already in Mail are lost and have to all be reimported?

No the old messages will not be lost upon importing new messages from your yahoo account. It will synchronize accounts, messages, and folders with your yahoo mail account.