Hi Peter,
I have been in a similar state, though I knew the end of "classic" Eudora was impending with Lion. I can't remember ever not using Eudora, and the program has the full history of every email I have kept and sent for nearly a decade. It's an important history for me for work, plus the features of Eudora are unparalleled. I hate to give it up
Currently I am running Apple Mail on my 2011 Airbook, and I've kept my 2010 Airbook with SL and continue to run Eudora 6.4 on it. I've been doing this for the past month and still feeling my way through this really serious issue. I have investigated every possible option (can't run SL on the 2011 Air), reviewed all other possible email programs, am testing Entourage, and Eudora OSE (it does work on Lion), etc.
The short answer is I have not hit on any good replacement for classic Eudora yet on Lion.
I have also considered giving up the new Air and Thunderbolt display (the reason I upgraded was because of a bad display), but I figure this is the future, need to adapt. But I think it is an idea with merit, at least till they get some of the Lion bugs worked out. THough I fear that will never revive Eudora as we have known it. Another losss with Lion - can't run Acrobat. This stinks because I used it often to assemble PDFs from multiple files - can't do that in Reader.
So why not migrate to Mail? It doesn't have the levels of control of Eudora or simple features, like opening the next unread message automatically, or advancing from one open message to opening the previous or next by using Cmd up or down, it doesn't display attachments in my sent box for some reason, I can't direct all attachments to one folder without manually saving every one, which changes the time stamp on the file too. You can't type a couple letters of someone's name or subject word to go to the most recent email to reply, can't open a message with the space bar, can't set different check email intervals for different accounts, I could go on and on - so many details.
Eudora OSE is not shaping up to be a full replacement either. At least it's heartwarming to see the Eudora icon on Lion! It's quite a different program from classic, somewhere between Mail and classic Eudora. There are actually features of Mail that may be better... still trying to work it out. So bloody time consuming to sort it all out. It's a real crisis.
In the end, I suspect I will just give up what I have had with Eurdora and go for a crippled alternative with Mail, unless someone finally comes up with a decent solution. I imagine there are a lot of hard-core Eudora users in a similar situation, anyway, you aren't alone and there are no good fixes as far as I have been able to determine.
It's disappointing to regret an Apple purchase - the first disappointment for me, and now with Steve Jobs gone... sad days.