Hi Tom.
Unfortunately this is a known bug. Your workspace's sensitive data are encrypted by the key which consists among others of your OS version. That's why upgrading your system prevents you from opening you previous workspace. We are planning on improving the decrypting method to be able to open the workspace also with the given password or something similar. Until then I am sorry to say there is no way to keep old workspace across system upgrade.
Anyway thank you for your input, it could speed things up a little bit. And we are glad to hear you like Toad for Apache Hadoop!
Once again sorry for the inconvenience.
Regards
Vladimír