As it says at the link I provided above:
"Only experienced Red Hat/Fedora Core users should attempt this, as the yum upgrade process is not officially supported and may cause problems."
While experienced users are familiar with safely and correctly resolving issues like package conflicts and dependency loops, inexperienced users can end up with an unstable system, and even lose data.
As Spooon said, it is possible to perform the upgrade. It's not recommended.