Hello everyone from Italy.
Forgive the long message but I am not good in explaining things in short in another language.
I use Mint 17.3 with kernel 3.19 (linux-image-extra-3.19.0-32-generic) KDE 4.13.2 and the oxygen theme.
After the news of the intel vulnerabilities, I read that no patch was coming for my kernel and I had to go to versions 3.13 or 4.4. I chose 4.4 and tried to upgrade it.
When I rebooted the machine, I immediately noticed that my graphic preferences had changed: screen resolution was higher, the panel different etc. So I restarted the machine with the previous kernel (that I hadn't touched), removed the new one and rebooted again.
Things were better (screen resolution and icons were ok) but still many things remained different: for instance when the mouse hovers over icons etc. a big notification comes up that previously I had taken down; the panel is semi-transparent and previously it was not; when I use alt+tab to switch windows/programs I have a banner with 3 images of the programs I can choose while previously I say only one at a time, I had chose to always see hidden files and now they're hidden, and so on.
A friend suggested that the kernel may have removed the graphic drivers, but I suspect that somehow it messed with the KDE preferences. What do you think?
Since I have a decent backup (I think), I imagine that if I overwrite the new changed files with the old configuration files I could avoid retuning all the preferences to my liking. But... which files could have been changed exactly??? Which should I overwrite? Which directories?
Thanks for every suggestion

In KDE 4, most of the config files should be in either ~/.kde or ~/.kde4
I would also guess that there may be something going on with the graphics drivers which may have triggered the change in resolution and possibly some of the other changes.

Just a quick upgrade: seems it's a kernel glitch that messes up X somehow. The only way to make it work at all is to add an attribute during boot that (in my pc) blocks the resolution to 640x480. Any other option makes the machine unbootable. Seems that others had the same issues with Mint 17.3
At least I recovered the old preferences, so I ditched the new kernel, restored the old one and will wait for a new release hoping the problem gets fixed.