This tutorial is about changing the UID of a user with an encrypted, by ecryptfs, home directory in Ubuntu.
You first need to logout the user you want to modify and kill all processes he runs. An easy approach is to restart and log in as root or as another sudo user.
To enable the root user (by default it is disabled) execute the following commands:
sudo passwd root
Now you can log in using root and the password you provided. Once logged in as root execute:
usermod -u newUID myuser
to change myuser's uid and
chown -R myuser /home/.ecryptfs/myuser
to change the owner of the encrypted files. You should now be able to log out and log back in as myuser.
Finally to disable the root account again execute:
sudo passwd -dl root