How can an existing COD (Cluster On Demand) image be modified for Bright 7.3 and Bright 8.0?

Basically by resizing the image and adding what is needed.

Required elements:
Bright COD image
Image manifest
XFS tools package (xfsprogs) must be installed
loopback device support (is usually enabled by default)

Step one:

gunzip the image
Example:
gunzip bcmh-centos7u2-8.0-5.gzip

You should end up with a file :
bcmh-centos7u2-8.0-5.img

Step two:

Resize the image to the size desired. By default the COD image has very little free space left. So, if you plan to add files of a significant size, then the size of the image needs to be increased.

Example:
qemu-img resize bcmh-centos7u2-8.0-5.img +10G # 
that would add 10 gigs to your image

Step three:

Connect the image to a loopback device:

offset=$(fdisk -l bcmh-centos7u2-8.0-5.img | grep Start -A1 | tail -n+2 | awk '{print $2*512}')
losetup --offset $offset -f bcmh-centos7u2-8.0-5.img

Step four:
Confirm the new size and the that the image is connected to a loopback device:

lsblk 
You should see a /dev/loopN in your device list.

Run kpartx on that loop device. You should end up with a partition when you run lsblk again, example :
kpartx -av /dev/loopN
lsblk
loop1       7:1    0    25G  0 loop
└─loop1p1 253:0    0    25G  0 part

Step five:
Mount and resize the filesystem of the image :

mkdir /mnt/image

Please note that loopNpN is supposed to be the partition that you got from lsblk after you ran kpartx.

mount /dev/loopNpN /mnt
xfs_growfs /dev/loopNpN

Step six:
Chroot and execute your changes

Example:

chroot /mnt/image
yum update -y
exit
sync

Step seven:

Umount and disconnect the loopback device

sync
umount /mnt/image
losetup -d /dev/loopN

Step eight:

Gzip the image and upload it using the instructions at https://kb.brightcomputing.com/knowledge-base/how-to-manually-add-cluster-on-demand-for-openstack-images/
Example:

gzip bcmh-centos7u2-8.0-5.img
export OS_PROJECT_NAME="bright"
cm-cluster-on-demand-openstack image install --is-public yes --image-file bcmh-centos7u2-8.0-3.img.gz