Doing a clean install helps reduce the cruft, and if you can go that way, it's probably a better path, but, opinions differ.
Most of your KDE stuff is going to be in a directory called .kde4
You should see it if you type (you need the a):
ls -la
...then, it's a matter of initializing each program, exiting the program, and then overwriting the important files with your own data....eg:
cp /mnt/old/home/you/.kde/share/apps/kabc/std.* /home/you/.kde4/share/apps/kabc/
You'll notice some things change, like going from kde3 to kde4, the directories change, like .kde4 is now the new location for stuff that was in .kde
...and you'll notice some applications have upgraded as well, for example, in Mageia2 Alpha, you have ~/.kde4/share/apps/kmail2 instead of ~/.kde4/share/apps/kmail
Sometimes installers miss stuff, so manually, you catch what got missed, although it takes more time
2011 Mandriva did a whole bunch of new experimental stuff, so it shouldn't be a surprise if you likely ran into problems trying to convert to Mageia