There are two good reasons for migrating old PDF email archives to PDF packages: to facilitate sorting and other functions, and to make it possible to add new email messages to those archives.
In PDF packages, each email message is converted as component PDF. You can then sort the messages by message folder, sender, subject line, date, size, or attachments. Also, you can create custom categories and sort by those.
You can open email archives you created with earlier version of Acrobat in Acrobat 8.0, but you cannot add email messages to that archive in the same way. If you try, messages will appear that will guide you through the process of creating a new archive from the old one and appending the selected messages to the new PDF package archive.