I tried this:

sed -i "s/jessie/ascii/" /etc/apt/sources.list
apt-get update
apt-get upgrade
apt-get install kde-plasma-desktop

Reading package lists... Done
Building dependency tree       
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 kde-plasma-desktop : Depends: kde-baseapps (>= 4:4.11.3) but it is not going to be installed
                      Depends: kde-runtime (>= 4:4.11.3) but it is not going to be installed
                      Depends: plasma-workspace (>= 4:5.0.0) but it is not going to be installed
                      Depends: plasma-desktop (>= 4:5.0.0) but it is not going to be installed
                      Recommends: sddm (>= 0.11) but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

And if I try to install mentioned dependencies one by one, eventually I get into dead end. Last time it was:

The following packages have unmet dependencies:
 upower : Depends: libimobiledevice4 (>= 0.9.7) but it is not installable

Package Suite: ASCII