This question already has an answer here:
To my astonishment, I found out I can't upgrade my CentOS 5.5 system to CentOS 6... (well I can, with a risky and painful process that is not supported and will most probably result in a mess)
So, rather that waste my time with a new fresh install that I will have to redo again and again every time a new major release comes out, I prefer to install a different distro and this time choose one that I know will always guarantee the possibility of a seamless upgrade to any new minor or major release.
Among the "big" linux distros, which ones take care of guaranteeing the ability to upgrade seamlessly across major releases? Does Debian?