I doubt that you can still purchase the 9.12 license, but regardless, 9.12 and all newer versions retain backward compatibility. Of course, this requires that you retain your private key if files are encrypted to your existing public key. I've never been a Eudora user, but the current email proxy should work fine with it for new email. If your old email is still encrypted, you will probably have to use the now included PGP Viewer to decrypt them.
You might want to consider trying an installation of the Trial. The software is the same - it is only the license that differentiates it from the purchased software level of functioning - you can just purchase and apply the new license if you are satisfied with it.
In addition to the Freeware license not allowing commercial use, the email proxy only processes incoming email. File encryption continues to function. There is no Netshare, Virtual Disk or Whole Disk Encryption functioning with this license. It also does not support Exchange servers. The Trial license is full functioning for 90 days.