The link you provided above is dead, so I have no idea what you did.

This would be the correct procedure to change from POP to IMAP. http://kb.mozillazine.org/Convert_a_POP_account_to_a_IMAP_account

Thanks. It appears I must have marked the option for deleting mails older than 2 months. I guess I thought my Local Folders were safe, and any change would be effective form than on. I wish Thunderbird had provided a warning message against retroactive change.

I had recently copied my C drive to an external, but for some reason, my profile was not copied (perhaps it was in use or maybe hidden files were not copied). A search for .sbd files, produced nothing.

Does Thunderbird make backups under another name, by any chance (hope against hope) ?

We need a tool to backup our mails. We need MozBackup to be a part of Thunderbird and Firefox. Not an external plugin but a great plugin integrated in our software.

Why the developer can't be happy about this? We're a team... when someone create something good... why don't give him a recongnition like a "meaningful" developer?

Thunderbird is GREAT... someone did money (postbox) with this core... Standing clapping at this?!?!!?

Does Thunderbird make backups

No, Thunderbird doesn't create backups itself. It's the responsibility of the user to backup the profile. http://kb.mozillazine.org/Thunderbird_:_FAQs_:_Backing_Up_and_Restoring

Thanks for the information and links.