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/