Mailing the story:
How to change default boot kernel permanently or temporarily on CentOS
When there are multiple kernels installed on CentOS, GRUB/GRUB2 boot loader chooses and loads one of them as the default kernel during boot time. The default kernel can be chosen differently. For example, CentOS can be configured to boot into the newest kernel installed (e.g., CentOS 6), or it can boot a last saved entry regardless of the kernel is newer or older (e.g., CentOS 7).