How to remove passphrase for a locked encrypted /home/user directory?

So, the main situation was described here. So far there is a laptop with Windows 8/upgraded Ubuntu Mate 18.04 in dual boot. Once the original encrypted /home/user directory was locked by a passphrase, I booted through "Live CD/USB", changed the password via "Try Ubuntu" option and since then Ubuntu Mate boots into a default "Try Ubuntu" 20GB desktop, with ~330GB of home directory locked under passphrase.
As is suggested here:
In short:

  1. Log in as root.
  2. Move the encrypted home directory so it mounts not on the home directory, but on a directory called Private in your home directory.
  3. Move everything out of the ~/Private directory into ~.
  4. Remove ecryptfs if desired.

I can't copy 330GB directory on the same 500GB laptop. Is it possible to mount it on a removable hard disk, do the necessary stuff, boot into original /home/user directory, remove encryption and copy this directory back to the laptop?

As I was planning to install ubuntu Mate 22.04 here, maybe during installation or upgrade
the old /home/user directory would be preserved and there would be a remove encryption option?

In the /home/user/.ecrypttfs there are files: auto-mount, auto-umount, Private.mnt, Private.sig, wrapped-passphrase. Maybe something can be done with them?
I actually currently boot under the host name, rather than user, so root is also under the host name.

I am not sure I understand your problem exactly so this may not be the answer.
Have you tried starting under recovery mode, dropping to the terminal and changing the passwords for root and the user with the locked home directory? That is how you reset a forgotten password.

I'll try, the point of the problem is that a passphrase is (probably) not a regular user/root password, which is required for a locked home/user directory.
After the original encrypted /home/user directory with the original Desktop were locked,
there are 2 files in the Documents folder for unlocking the directory. And they both require "Enter passphrase", not password. This should be related to how /home/user directory encryption works (chosen upon installation).