Well i just did the update on my AMD CPU/GPU Laptop with Manjaro/KDE, almost everything is working flawlessly, besides some unsharp fonts from shortcuts on desktop and light distorted fonts inside “system settings” menue . If this are the only problems, then i hope its the same for my Intel/nvidia PC.
All what i did was (Timeshift snapshot, just in case):
1.Fresh start
2.Log out
3.crtl+alt+f3 to open TTY
4.sudo pacman-mirrors --fasttrack
5.sudo systemctl stop sddm
6.sudo pacman -Syu
and spammed Y to replace everything
7.reboot
Im NOT even deleted ./cache or reseted ./config stuff, on my 4year’s old home user account. I’m not even switched any of my Theme to default, that i downloaded from the KDE Store, everything is still customised, only a few icon’s (~1%) are missing. (I’m not using Virtual Desktop or Widgets btw. maybe there are problems? idk)
My Global Theme “Layout” just disappeared (or was deleted, while i updated?), while i still had tranparency/blur effects for my Taskbar/Terminal as before with KDE Plasma 5, so it must be active 
Edit: My AMD Laptop had performance issue with X11, switch to Wayland in the Login Menue and after that my Window Dragging was smooth again. My Intel/nvidia PC is running fine with the new Plasma6 Update under X11 and gaming working pretty damn good, i love this update, thank you @philm and the other Maintainer’s. 
Yeah i had to put my desktop icons/shortcuts at the old spotts again.
I fixed the floating taskbar with rightclick on the Taskbar and “Enter Edit Mode” and clicked on the “Floating” switch. DONE