After updating the O.S. when starting Plasma you can observe following
behaviour:
a) desktop background if set if lost. It appears that the picture files are
there, however cannot be seen by the system. If you download them again and
install them you are prompted if you want to overwrite the file, it therefore
exists. You confirm but the picture file remains invisible and cannot be
selected. You do not confirm the same happens. Thus the photos that where used
beforehand are lost as background and cannot be reinstalled afterwards. This
"survives" as problem also if you logout the session and login again. 
b) icons form third party are shown in the list but partially not used. If you
set the icon settings to breeze, it will say yes, but eventually will not
honour the settings (in particular if you have the folder view with desktop and
trash from 42.1 on the desktop left. 
c) The Meteo plasmoid of 42.1 is taken over but presents a malformed display
with the temperature displayed outside and far from the plasmoid itself. 

Mouse cursor no problem. Good.
Akonadi started O.K. and application seem to work. Good.

Screen resolution: the resolution (during update on virtual machine kvm) was
NOT taken over from previous install which cause the task bar (set to hide
automatic) to fail. This is not dramatic as you can call the system settings
via the run dialogue and you can set successfully a suitable resolution (and
the bar turns) but this can be a challenge to "I just am a user" users or to
first timers that installed the previous O.S. first and think to migrate in
about a few weeks after release. So this would be good to be looked after.