Thunderbird imports and changes nothing to do with Eudora, but two things. It imports all your Eudora mail, so if you keep using Eudora you can not import your changes. Eudora import does requires that there be registry keys set by Eudora on the machine, so you can not migrate the eudora data and import that, the import must initially occur on the device with Eudora installed and then Thunderbird data needs to be moved.