Okay, the results
The procedure worked almost as outlined. I could not resan the server (5) but instead I clicked properties and followed the wizard with default options. This installed Veeam software.
Similar, I had to edit the extents one-by-one and (with default options). This installed the required agents for the mount server.
After this I could rescan the repository and everything looked ok but a test backup failed:
Unable to allocate processing resources. Error: No scale-out repository extents are available.
After a few attempts to figure out what went wrong I finally decided to restart the Veeam services and that did the trick