I had a similar problem with 6.
I went back to 5.2.1b2 to be exact.
First thing is to go into the system folder and take the Eudora folder out and put it somewhere else. Your mailboxes and other things you may want to save are in there.
You can then throw away the Eudora Application folder which is probably on the main level of your booting volume. Then download and install 5.2 fresh. Once that is done you can manually transfer your mailboxes from the saved folder into the new Eudora folder that was created in the system folder. Don't get the two folders mixed up, one has the application and plug-ins,etc. and the other is in the system folder with your mail.
Aside from your mailboxes, you will also want to transfer the Eudora settings, nicknames, etc., in fact you could probably just move the whole folder and replace the one that is there but I don't remember if there is anything 6 did to alter the actual folder name that would make 5 not recognize it. If you do it piecemeal don't bother to move the ads folder or the cache.
It really isn't difficult as long as you save your mailboxes, settings and nicknames (addresses) to put in the new install. If your special dictionaries are normally in that folder make sure they are put back, too.
I had no problems at all reverting. To be extra safe, make a copy of your Eudora folder (from the system folder) and put it away somewhere or on an external location. That way you will not lose your saved mail and addresses regardless of what happens.