To change the first distro, you can just install straight over the top. Mandrake may offer the choice of upgrading if it's a later version.
I've never managed to have more than one distro running without having to manually modify the bootloader (lilo, by preference).
Whether this is because I usually tell it to format the various partition(s) or not ???
Also the Mandrake 10 that's available is the "community edition" - this means that it's a testing version. The final one will be called "Official".
You'd find a few bug's but it seems to work pretty well for me. I just check for updates every 2 or 3 days (bugfixes, security and normal updates).
I think that with mandrake you'd have to re-download the latest or whatever version you want and then either upgrade or install afresh.
If you want a distro that you can do a "dist upgrade" then debian and gentoo will both do this. I would suggest that if debian "presses your buttons", then get knoppix and install it to hard drive - you then just manage it as debian. "Proper" debian can be quite an effort to install, because lots has to be done manually. Gentoo on the other hand, is quite straight forward to install - the install doc's are very good, but the downside is that it can take quite a while - being source based, it likes to compile everything for your machine. You'd have to get the latest 2 disc version then probably the "stage 3 + GRP" would be the quickest (the GRP bit is pre compiled packages - if you did a direct install of say KDE, it likes to download and compile the various packages that make up KDE one at a time - the first time I did that, it took 15 hours).