I'm still trying to recover the mail that I had stored in kmail and that seems not to have made it over correctly when I migrated to kmail2. I saved the entire hierarchy of old mail folders off to the side as a separate maildir account, effectively quarantining it, and backed it up, That way I can work on it without corrupting my current mail, without losing anything, and with the ability to track my progress.

The problem appears to be that my old files are a mixture of mbox format and maildir format, resulting from a long history of mail migrations. kmail seems to have been able to work with such a mixture, but kmail2 apparently can't. I think what I need to do is to convert all the old mbox files to maildir directories in place (please correct me if I'm wrong). I'd rather not process the mbox files one by one if I can avoid it, since there are a lot of them and they're scattered within the directory structure.

I expect I'll have to work from the command line, not from kmail2, to fix the problem. Can anyone advise me as to how I can untangle this mess? Is there perhaps a wiki article on it somewhere?

Please try the "KMail Mail Folder" and see how that works - it is supposed to be compatible with KMail 1 mail folders.

I'm confused -- where do I find the Kmail Mail Folder?

It is a type of "Recieving Account", which is compatible with KMail 1 folder setups. It should be used instead of either Maildir or MBox formats when dealing with a folder tree used by KMail 1.