I recently upgraded one of my main desktop systems to Opensuse Leap 15.1. After the upgrade I got some difficulties with the Nvidia driver and sddm. I suspected sddm to be the reason - so I wanted to change the "displaymanager" for a test. I was used to doing this by changing a respective entry in the file "/etc/sysconfig/displaymanager". But this had no effect!
The reason for this failure was something I missed already at the time when I upgraded from Leap 42.3 to Leap 15.0:
SuSE uses the "update-alternatives" mechanism since Leap 15.0. You easily get information from SuSE web-sites how to use this mechanism. See e.g.
However, I wanted to get some background-information about update-alternatives, e.g. about which files or directories are used to keep related information. So, here are some links which may help others, too:
https://en.opensuse.org/openSUSE: Packaging Multiple Version guidelines
In short: update-alternatives provides the possibility to easily define and switch between different programs for the same (or a very similar) purpose on a Linux system. It works via symlinks, which are defined in the directory "/etc/alternatives".
As expected you can use "update-alternatives" on the command line. However, on an Opensuse-Leap-system yast and yast2 can be enhanced by a package "yast2-alternatives". After installation you find a respective entry below the main point "miscellaneous" of the yast2 menu. This allows for quick changes between already defined alternatives - e.g. for the desktop session, the displaymanager, java, ftp and others.