I've done the same you're now aiming in the following way..
1 - backed Up all my data, including MySQL dump to my databases;
2 - resized my linode and created a new disk;
3 - created a new configuration profile for second disk and did a fresh install of CentOS 7;
At this time I can boot my Linode with CentOS 6.2 or CentOS 7... It meets my needs beautifully!
For steps 2 and 3, see here: https://www.linode.com/docs/migrate-to- ... n-profiles
For fresh install, just go thru the Getting Started Guide as it includes centOs7 install: https://www.linode.com/docs/getting-started
Hope it helps,