Well honestly, I've seen people who have managed to completely change the OS. For instance, they get a RH9 box, and less than a week later, it is gentoo. so if you do keep up with ALL the changes, it really won't be that much different, in my opinion. but as liliafan said, it is much easier saying I run <insert favorite distro here> V. x.0
Comparing RedHat to Gentoo is like comparing Mandrake to FreeBSD. They both use different packaging methods, different libraries, and have an altogether different directory structure (in some areas.)
I'd tend to think that the person started off running RedHat, then switched to Gentoo (or whatever) not by updating libraries and recompiling a kernel, but by installing the new distro in place of the former.
As far as I know, the only O/S that allows you to upGRADE to a newer version (completely, not just by updating files) are the BSDs - I've done it on FreeBSD, but haven't tried any of the others yet. You can start out with FreeBSD 4.6.2-STABLE and, after you're finished, be running 4.9-RELEASE. With Linux, it's a bit different, but essentially, if you upDATE the libraries, applications, and the kernel, you can wind up with the equivelant of a newer version. Keep in mind that there are some things that just won't run on a newer version, regardless of what you upgrade or update.
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OK, I think I've gone off subject far enough now...
I started on Slackware 2.4 (I think - was definately 2.x) and now have 2 servers running Slack 9.1, plus my laptop.