wiki

Leapp is a tool that allow an inplace migration Ex from 8 to 9

Prerequisite packages:

leapp
leapp-upgrade-el8toel9
pciutils
systemd-container
leapp-deps
leapp-upgrade-el8toel9-deps
python3-leapp

Create a folder /tmp/packages on offline system, copy all the rpms to /tmp/packages. Install the packages.

# mkdir /tmp/packages
# cd /tmp/packages
# yum localinstall *

Then you need to download the ISO from Redhat 9 and start the precheck and then the migration

# leapp preupgrade --no-rhsm --iso /path/to/RHEL9-Binary-DVD.iso
# leapp upgrade --no-rhsm --iso /path/to/RHEL9-Binary-DVD.iso

http://wiki.centos.org/TipsAndTricks/CentOSUpgradeTool

http://liquidat.wordpress.com/2014/07/17/howto-upgrading-centos-6-to-centos-7/