RHEL-7 switched to using grub2 as the bootloader, while RHEL-6 used yaboot, so the upgrade-tool must convert the existing yaboot.conf to config file understood by grub2. And also switch the bootloader package in the system being upgraded.
redhat-upgrade-tool does not change your boot loader. You can do that on your own after a successful system upgrade, but to minimize risk of an unbootable system, redhat-upgrade-tool leaves the boot loader as-is.
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