Generally, upgrading is a hacky way of getting to a new release.
It is far simpler, easier, and all in all a better option to re install the newer version..
Upgrades tend to leave new packages out, leave old packages behind, and 99 times out of a hundred generally just screw things up.
I would recommend backing up your data, and re installing.
If you use a distro that runs a fairly short release schedule (such as Fedora), i would also suggest keeping /home on a seperate partition/drive, and keeping a list of the packages and repo's that you install and use.
Then installing a new version becomes quick and painless.
- Install new version, using the existing /home partition
- Do the usual yum update etc.
- Add the list of repo's and
yum install `cat packages.txt`
- Bobs your fathers brother
This has worked for me successfully since Fedora 12.