Using kde, you could try use application Screen resize & rotate (under system menu).
Run that program and make sure it reside in system try and set to desired resolution.

Then run Control center ----> Peripherals--->Display---->tick on Allow tray application to change start up settings and tick Apply settings on KDE startup.

Do this for each user.

But...there is a problem, when user using lower resolution log out/off, the resolution will follow with the next user if the other user is using higher resolution. It could be my own problem, because I use very old monitor.
The problem will not happen the other way ( with higher resolution to lower resolution).

The workaround is to kill X or to restart X after user with lower resolution log out.