What is the account type - POP or IMAP?

Do you have a backup?

IMAP. No backup that I'm aware of.

TB-profiles (which have all emails, contacts, etc,etc) are here: "%APPDATA%\Thunderbird\Profiles\" that usually is: "C:\Users\<WindowsUserName>\AppData\Roaming\Thunderbird\Profiles\" inside that there are different TB-profiles, like: xxxxxxxx.<profile-name> make a backup of entire "...\Profiles\" first , before you do any other steps.

TB = Thunderbird.

this commandline allows you to choose a <profile-name> : 64bit TB users: "C:\Program Files\Mozilla Thunderbird\thunderbird.exe" -P 32bit TB users: "C:\Program Files (x86)\Mozilla Thunderbird\thunderbird.exe" -P

then choose your older profile, all old stuff & account will appear in TB. do not do anything in old mail-account in TB. add new mail-account in TB, for the new mailserver, once the new one is working, then you can copy/move older emails from old mail-account into new mail-account in TB.

I can find only 1 profile folder, and it seems to be the one in use. I had already updated my account info without backing up, thinking anything already on my hard drive would be retained. It appears I was wrong, I guess? Is there any other way I might still have legacy data?

IMAP. No backup that I'm aware of.

Instead of changing the server settings of the existing IMAP account, what you should have done is to create a new IMAP account for the new server. Now it's too late. What you saw is basically a demonstration how IMAP works. With IMAP all mail is managed on the server, and each client accessing the account is a mirror of the data on the server. When you connected to the new server the first time, it didn't have any mail yet, and hence it wiped out all messages previously stored on the old server.

Without a backup I don't see how you can recover your lost messages. Your only chance would be to gain access to the old server again, and copy the messages off of the server to your Local Folders account. Not sure if that's still possible though.

For the future, make sure to create backup of your Thunderbird profile on a regular basis. https://support.mozilla.org/kb/profiles-where-thunderbird-stores-user-data#w_backing-up-a-profile

Since you're using IMAP you'd also have to download an offline copy of all your messages on the server to your local disk, so that there is something to be backed up in the first place. In Thunderbird speak that's called 'Synchronization'. https://support.mozilla.org/kb/imap-synchronization

look into your server. if you have deleted your emails there too, during migration, then its gone poof, unless you run data-recovery software on server storage. if you have not-deleted your emails in server, then place them back under your mail-account in server, then TB will be able to show it.

and sometime its possible to recover data also from windows10 computer, when you had lots of space empty in your storage , so your OS file-system did not need to overwrite old data yet, then data-recovery software in win10 will/should be able to show all deleted files.

its also possible that, the single profile that you now have , contains the older profile within it, look for .bak files.

copy this profile into another folder, rename bak files into their correct corresponding extension. you may also have to correct some settings in "pref.js", then use a second-TB to use that copied "profile" folder . (option-2 in linked page)

bak files will give you other data, but not old-emails.

here is a list of data recovery software.

please wait for my last post to appear on top of this message , that needs to be approved by a TB Mod/Admin.

Thank you. I haven't had time to get back to this issue, but I'll look at each of these suggestions and let you know how I fare.

No joy on the server or the .bak file.

I did find something interesting, and I hope promising. Within the the profile folder there are 2 imap folders, each containing what looks to be the folder structures I used for those accounts, and they have 2GB of data between them. Is there a way to recover these?

please make a backup copy of that too again, then you MAY do these:

as far as i understand, once you try to use that 2GB worth of IMAP based local data with a IMAP based mail-account , then one of these will happen: result-1 : if the IMAP mail-server itself does not have the EXACT SAME EMAILS (which are in that 2GB amount of data) , then immediately after you check for mail (from TB mail-account) , all those emails will be gone. result-2 : if the IMAP mial-server itself contains all emails EXACTLY SAME AS THOSE 2GB DATA , then all emails will be back instantly or after a sync event.

if you get result-2, then you will probably not be here reading this ;)

to prevent what happens in result-1, you may do this: manually create a second imap mail-account in TB, use a slightly different username & slightly different server name , (so that this mail-account cannot sync with remote mail-server) . By using the config:editor (Config Editor) you have to find out what exact account name & folder name this second imap based mail-account in TB uses , then copy those 2GB worth data inside the specific folder of second imap based mail-account , then start TB , it MAY or may not show all emails inside second mail-account. if it does, inside TB, copy them to a sub-fodler under the Local Folders. you suppose to do these in a Portable-TB, where there are no other mail-account exists.

mozillazine website shows more/correct detail on, how imap mail-account in TB uses different sub-folder & file names inside the TB-profile, to represent+hold emails of a mail-account. that will give idea, how or what need to be renamed.

above steps, are done here in a slightly different way.

sometime a Windows computer may keep CACHE copy of various data inside "%APPLOCALDATA%" folder, check for Thunderbird cache there too , it may also be inside another folder , i don't remember well now. ( in macOS , there is a separate CACHE category/type of folder for TB (and other apps too) , so similar functionality may/should also exist in Windows , where various TB data still exists )