Yeah, you can't change the username if that user is logged in. Basically what is needed to rename/move is:
sudo passwd root [to set the root password]
logout of your account [ie. pine64]
log in to root account [using newly set password]
usermod -l new-login-name -d /home/new-login-name -m pine64 [renames pine64 to new-login-name, moves home directory]
logout out of root, log back in as new-login name
run sudo passwd -l root to lock the root account again if you wish. use passwd (without sudo) to change the password of the newly renamed account if it is still pine64
sudo passwd root [to set the root password]
logout of your account [ie. pine64]
log in to root account [using newly set password]
usermod -l new-login-name -d /home/new-login-name -m pine64 [renames pine64 to new-login-name, moves home directory]
logout out of root, log back in as new-login name
run sudo passwd -l root to lock the root account again if you wish. use passwd (without sudo) to change the password of the newly renamed account if it is still pine64