Plasma 6 landed in opensuse. However, I recommend to do it in tty because if you do it in plasma the upgrade will be incomplete because the shell will crash in the middle of update without installing all. In case this happen to anyone, just reinstall plasma 6 pattern and it will go without problems
How exactly did you do it? Any links to instructions and commands?
First logout of Plasma, then you can change to tty 4 (for example) with ctrl + alt + F4, then you can login there with your user and do the upgrade safely in that other tty (or another one if you want to haha, just not in the normal desktop).
I think the Plasma desktop is always in tty 3 (ctrl + alt + F3) and the Plasma login is in tty2, in case you want to go back.
Screen or tmux are awesome to ensure that you can get back to a running shell. Like over SSH or on another tty.
In my case I had another WM installed (iceWM, I think it was there by default?) and did the upgrade from there. Unfortunately it does seem that if you try to upgrade from within KDE it will crash part-way (I used zypper dup and it failed).