mails with attachments and levelled labels

What are 'levelled labels'?

Unfortunately Thunderbird duplicates them instead of labelling,

Labels are a Gmail concept. Thunderbird translates labels into folders. What does 'Thunderbird duplicates them' mean?

Drag and drop method won't do with such large ammount of data

Don't use drag 'n drop. Instead select the messages to be copied, right-click a selected message - Copy To. Pick the destination account and folder.

Do copy messages in chunks, not all at once.

In any case, disable your anti-virus software while messing with such large mail files.

I tried manual files replacement in profiles folder

I don't understand what that means.

What are 'levelled labels'?

tree structure - label underneath other label, like folders in folders, with multi-level complexity

Labels are a Gmail concept. Thunderbird translates labels into folders. What does 'Thunderbird duplicates them' mean?

Creates duplicates, copies of copies because label->folder translation creates copies instead of referrences. Same mail in: "all", "important", "starred", "label x", "label y"; copied within merged-mail-file (those without file extension).

Don't use drag 'n drop. Instead select the messages to be copied, right-click a selected message - Copy To. Pick the destination account and folder. Do copy messages in chunks, not all at once. In any case, disable your anti-virus software while messing with such large mail files.

same thing - non-reliable method, unless done in small packs (500 mails from 22 000, mulpipled by x labels, is small pack) and constantly supervised (copy process likes to quit unexpectedly and you don't really know what's been copied. "Copy Folder" add-on gives a better solution (has counter and doesn't break), but equally slow, and doesn't manage that huge ammounts of mails in reasonable time (hundreds of hours is not reasonable time).

>I tried manual files replacement in profiles folder< - I don't understand what that means.

I went into the profile file structure and copied all from account xxx to account yyy (...)\ThunderbirdPortable\Data\profile\ImapMail\imap.gmail-x.com to (...)\ThunderbirdPortable\Data\profile\ImapMail\imap.gmail-y.com.

When done in offline mode, it works perfectly, but vanishes while synchronising, as I posted in original help request.

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English isn't my first language but I've got a feeling that I've got to post same message twice. I very and I mean VERY appreciate your help, but I feel like being on interrogation considering can I even know the difference between folder, label and file etc. If I'm wrong, please forgive me. I think of myself as quite a poweruser (but only - user) in windows environment and I know a bit about different systems, scripting and other non-basic stuff, but I have absolutely no idea about Thunderbird js commands handling and structure - that's why I asked help :) - maybe someone already knows the solution and might share it. I think the answer lays in "Copy folder" add-on which makes operations I need, but slooowly and one-by one (mail), instead of on marged mail files. It's a matter of "registering" mail files and folders under target profile so that they become more important than those on server. To become uploadable. Is it even doable in rather painless way?

Have you tried this?

It would seem useful to eliminate your email client from this large data transfer. Email protocols are designed for "little and often" and don't cope well with large blocks of data.

Yes I tried. Whole process is a pain - mainly (i think) to convince people to buy more space (which from google pov is best solution and it's hard to be surprised). And it would erease my label info which is somewhat crucial but I tried it anyway, just to check if it works. First I tried "Import email and contacts" - gmail don't supported as source (at least for me - I've got alert telling me that). Second I tried "check email from other accounts with Gmail (POP3)" - I've had to do several security downgrades in settings, got some messages about "possible password leaking" or access attempts blocking, found on forums to find captcha confirmation screen (which is not mentioned in any google migrating tutorial, and finding it wasn't easy - clicking direct link on some forum would be rather irresponsible in that case). On the end I've got message that in this particular case I have to turn off "leave copies on server" option. God knows why. But I know why - to convince me to buy more space, which I'd even consider I it wasn't simply a backup which I surely erease in a few months. And I don't like just to give up and pay instead of learning how to do things right.

I'm not sure if there is one exact way to do it right.

tree structure - label underneath other label, like folders in folders, with multi-level complexity

I'd probably call that nested folders.

Creates duplicates, copies of copies because label->folder translation creates copies instead of referrences.

Thunderbird is not supposed to do that, but you aren't the first one complaining about it. You may look for existing bugs in Bugzilla, or raise a new one if none exists. https://bugzilla.mozilla.org/

The pragmatic way would be to unsubscribe from those somewhat 'redundant folders' and delete the corresponding mbox file in the profile.

same thing - non-reliable method, unless done in small packs (500 mails from 22 000, mulpipled by x labels, is small pack) and constantly supervised

I don't disagree. In any case, the mail server probably has it's share for this process being slow and unreliable. I don't know another way to do this, even though it's tedious and a real pain.

When done in offline mode, it works perfectly, but vanishes while synchronising, as I posted in original help request.

This is the way IMAP works.

English isn't my first language

It isn't my first language either.

I think the answer lays in "Copy folder" add-on which makes operations I need,

I never used that add-on, so cannot comment on it.

This is the way IMAP works.

Yes and no: when I use copy paste/drag and drop/Copy Folder add-on method, changes are uploaded. In offline mode mentioned methods don't work (greyed out or no reaction). So to conclude - changes to work, must be done in online mode. It doesn't matter "on which side" (client or server) they're made, only connection while changing matters.