I recently purged KDE from my Linux Mint 17.3 KDE edition and installed Cinnamon.
I was able to upgrade it to version 18 (Sarah) Cinnamon. Yes, despite it is KDE edition. There were problems, but I was able to solve them.
RELEASE=18 CODENAME=sarah EDITION="Cinnamon 64-bit" DESCRIPTION="Linux Mint 18 Sarah" DESKTOP=KDE TOOLKIT=QT NEW_FEATURES_URL=http://www.linuxmint.com/rel_sarah_kde_whatsnew.php RELEASE_NOTES_URL=http://www.linuxmint.com/rel_sarah_kde.php USER_GUIDE_URL=`yelp help:linuxmint` GRUB_TITLE=Linux Mint 18 Cinnamon 64-bit
Note, that I changed KDE edition to Cinnamon here. But that is less important.
What interests me is what is TOOLKIT=QT? And how do I change it to GTK?