I use Arch + KDE Plasma on my desktop computer and EndeavourOS + KDE Plasma on my main laptop (Wayland on both). However, after upgrading KDE to version 6.7 on the latter, the login screen continues to display the Breeze theme from version 6.6. In other words, the login screen hasn’t been upgraded to version 6.7. (I don’t have this problem on my desktop, which runs Arch.) How can I fix that? Thank you.
Try System Settings → Login → Apply Plasma Settings → Reset to Default Settings.
Thank you. Unfortunately, it didn’t solve the problem. I still see the previous image (from Plasma 6.6) on the login screen. Once I have logged in, I see the correct image (Waterfall) as my wallpaper. It’s as if the Waterfall login image isn’t available, so Plasma defaults back to the previous version’s image.
I got the login screen to change by applying system settings to login screen under that same settings section that @Schlaefer referenced, so give that a try
Frustrating though, took me a few reboots to figure this out, and not sure why PLM didn’t update automagically like the desktop and lock screen 
[solved]
Root Cause
The Plasma Login Manager (plasmalogin) maintains its own separate copy of wallpapers under /var/lib/plasmalogin/wallpapers/. When EndeavourOS was initially installed, it copied the default KDE wallpaper (“Next”, which displays as “Waterfall”) into that directory.
The problem: this copy wasn’t updated when the plasma-workspace-wallpapers package was upgraded. The system-wide wallpaper in /usr/share/wallpapers/Next/ was updated to the KDE 6.7 version (June 16), but the plasmalogin copy remained at the old KDE 6.6 version (May 13). This is why the desktop and lock screen showed the new Waterfall wallpaper correctly, while the login screen still showed the old one.
Additionally, the KCM (System Settings module) for Plasma Login Manager has a bug in KDE 6.7 — its QML is broken (missing properties like cfg_PreviewImageDefault), so the Apply button does not actually save any changes. The KCM also displays a “Waterfall” preview that does not match any installed file, which adds to the confusion.
Solution
Replace the stale copy in /var/lib/plasmalogin/wallpapers/ with the current system version:
Backup the old copy (optional)
sudo cp -r /var/lib/plasmalogin/wallpapers/Next /var/lib/plasmalogin/wallpapers/Next.bak
Remove the outdated copy
sudo rm -rf /var/lib/plasmalogin/wallpapers/Next
Copy the updated wallpaper from the system
sudo cp -r /usr/share/wallpapers/Next /var/lib/plasmalogin/wallpapers/
Fix ownership
sudo chown -R plasmalogin:plasmalogin /var/lib/plasmalogin/wallpapers/Next
Restart the login manager
sudo systemctl restart plasmalogin.service
Verification
Before the fix:
# Old copy (May 13, smaller files)
$ ls -la /var/lib/plasmalogin/wallpapers/Next/contents/images/
-rw-r--r-- 1 root root 8287996 May 13 09:07 5120x2880.png
After the fix:
# Updated copy (June 16, larger files)
$ ls -la /var/lib/plasmalogin/wallpapers/Next/contents/images/
-rw-r--r-- 1 plasmalogin plasmalogin 8735616 Jun 16 10:31 5120x2880.png