Where do I need to change "Ubuntu" when naming my own custom distribution?

Here's my two cents about this:

You can try to change the name of the distribution but be careful, if you change something wrong, you might encounter problems while installing or later when you will use it. But if your absolutely need to change it here's what you can try:

The two you provided are correct (you can also update /etc/issue.net just to be sure). The other thing you can do is this:

Open the ISO (ISO Master) file and go to the folder called .disk. In there is a file called info. The release name is there and can be changed. view this post.

This should change the name of your distribution.

If that does not work, you can change the following files:

NOTE: the location might change depending the version you are using and file might be not found as well

  • /boot/grub/grub.cfg
  • /etc/grub.d/00_header
  • /etc/grub.d/05_debian_theme
  • /usr/share/yelp/ (all files & all sub folders)
  • /var/run/motd
  • /etc/motd
  • /etc/update-motd.d/10-help-text
  • /lib/init/rw/motd
  • /etc/gdm/failsafeXinit
  • /etc/samba/smb.conf
  • /usr/share/gnome-about/gnome-version.xml
  • /usr/share/pyshared/usbcreator/install.py
  • /usr/bin/grub-mkrescue
  • /usr/lib/grub/i386-pc/config.h
  • /usr/sbin/grub-install.real
  • /usr/sbin/grub-mkconfig
  • /usr/sbin/grub-mknetdir
  • /usr/sbin/grub-reboot
  • /usr/sbin/grub-set-default

good luck!