[CentOS] Upgrading to CentOS 6

I recently built a CentOS 6 system as my main machine at home.
With a bit of help from members of this list, it is now working
better than the machine it replaced (RIP).
The new machine works so well, that I would like to convert some
CentOS 5 machines to CentOS6.

I did some research on the web and the new install is still considered
the proper way to upgrade CentOS.  Same as Fedora and RHEL.
The question becomes, "What makes the Ubuntu developers so
clever that they can do major upgrades through their apt based
update system?"

There must be some sort of gotcha or tradeoff involved in allowing this.

Does anyone have any insights on why they can get away with this
while CentOS cannot?