Hi there,
I recently tried KDE as I could not get fractional scaling on Gnome to work properly. So far I like it very much, especially fractional scaling is a must have on a small HiDPI screen.
My only question is: Can I make scaling change during runtime instead of reboot? This was something that worked in Gnome, where I instead had problem with fractional scaling. 
Or is this just something where I cannot have it all, fractional scaling and live changes to scaling?
Thank you!
It’s impossible, but you can change size of font, icons, buttons and other things instead – some may ned app restart. Even if that’s not solution, it’s always alternative 
You have to restart X for the change in scaling to take effect in KDE. One thing which kind of works on the fly is changing the DPI in the font settings instead - it only requires restarting the app. However, it is super finicky on its own and tends to break stuff in KDE. So yeah, it’s basically restart X or bust in KDE.
I am personally dealing with this by relying on the sesssion save/restore, which works quite well in KDE and makes restarting X less painful.
Something like this might work:
kquitapp5 plasmashell && kstart5 plasmashell </dev/null &>/dev/null &