P.S.: Migrating decades of local messages was a huge problem. It's been so long that many import/export tools are defunct. I was even loading up extremely old versions of Thunderbird to see if I could import to that and then upgrade.
However the GitHub edition (not Sourceforge) of Eudora2Unix [0] saved me and deserves a shout-out here.
It took some tweaks and a harness to repeatedly try the conversion, since I wanted something I could literally drop into the Thunderbird profile folder. The final result wasn't perfect, (some file attachment issues linger) but it's way better than having it all at risk of Eudora.exe just refusing to launch one day.