by Howard Miller.
I'm out of my depth unfortunately being neither a Centos or a Plesk user. Even the latest build of Centos 6.5 doesn't come with nearly new enough versions although I do know that there are some reasonably reliable RPM sites that provide the required versions.
In general, upgrading things like PHP and MySQL can be potentially challenging.
Unfortunately, I wouldn't know Plesk if I fell over it and have no idea how that might affect things.
However, if you do manage to get them upgrade you should have no concerns about your existing sites. They will (should?) be fine.
As always, take backups before you break it