The best way to do this is to install the KDE version of Mint in a dual boot with the main edition until you are comfortable with KDE, then you can get rid of the main(Gnome) edition. Installing KDE on top of Gnome is hard to do without some Gnome attributes bleeding over into KDE.
If you are determined to leave Gnome, you might also check out the Xfce and LXDE versions of Mint. They are lighter than either Gnome or KDE and boot and run somewhat quicker on most machines.
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