Officially, upgrading the KDE version of Linux Mint 17.3 to 18 is not supported, because "Linux Mint 18 KDE will use a new and different desktop called Plasma". I presume this means that the KDE4 to KDE5 upgrade process is not trivial. The official way to upgrade is to do a clean reinstall.
I use Linux Mint on my main (scientific computing) workstation, and would much rather not do a clean reinstall: following advice, my
/home/ is not on a separate partition, I have several different (non-physical!) users installed, and the official installers just want to wipe everything and start again. I'd really much rather not.
What I'd like to do is update all of the apt sources to point at the right place, do an
apt-get update && apt-get dist-upgrade and go home for the evening. As it stands at the moment, quite a few things randomly break every time I do an
apt-get update (at the last one, nvidia drivers, cuda, and xfce due to a change in dbus!). I like the look of Plasma, and the live usb of 18.2 runs great on my computer. I want to upgrade without wiping everything. This is far from an unreasonable request.
I have't heard of anyone else doing this successfully for KDE Mint 17.3, and I'd like to know if it's possible.
mintupgrade certainly thinks not. I'd like to try.
What happens if you just do something stupid, like
sed -i 's/trusty/rosa/' /etc/apt/sources.list?