Following the documentation of @Benjy (see here) I tested a migration from OH2.x to OH3 with milestone 1. As it was a very old RasPi I had to wait a lot, but it worked.
To be faster than with the old RasPi I repeated the tests on my desktop, which is a x64/openSUSE box. openSUSE uses RPM packages, so that covered an additional test scenario as well.
The main information: a migration on openSUSE is NOT possible at the moment. The reason is a dependency between the openhab2 package and the openhab package I believe. Using YAST the admin tool recognizes the dependency and removes the openhab2 package before the openhab package (=OH3) is installed. This removal includes all user generated files as well. Everything that is located in the folders created by the package installation is removed completely. Hence nothing to take over from the older installation.
I do not know whether this behaviour is openSUSE specific or is something that depends on RPM packages.
Be advised you need to copy/backup your configuration files and other data (rrd4j database files e.g.) before you switch to OH3, at least on openSUSE and with the packages as they are atm.
I don’t think the situation can be fixed on the OH side as the packages share at least the user “openhab”, the group “openhab” and the OH2 service files need to be deleted somehow so they do not interfere with the OH3 ones. Which leads to the dependency between the installation packages as a must. But perhaps there is someone out there who knows better than me and can suggest how this situation can be fixed.
To accomplish the test I really wanted to do, in a second attempt I removed the openhab2 package myself but recreated the user and group (with the same uid/gid as the openhab2 package), recreated the folders for configuration and logging and placed my config files in the proper locations, taking care that the file permissions were correct. And then installed OH3. The results matched the ones in the RasPI-tests linked above.