Hello jzelle,
There is no fee for migrating to SmartMail from cPanel Horde, provided your account is on a cPanel server. Yes, you will lose all old emails on the mail server, so you should backup any emails you wish to keep. You would need to recreate the email accounts on the Smartmail interface.
Also, keep in mind that you *may* need to update server settings for your email clients like outlook.
If you are interested in switching, you can contact our support department about it.