@Klojum: Yes, indeed, many moons ago. And that is as I think it should be: Install a system (here mediaplayer) and use it for years for its designated purpose, without thinking too much about it. Sorry for not being more clear about my plans: I meant migrating, i.e. new installation but with the user files saved.
@chewitt: Thanks, that helped! It is even more moons ago that I used a unix shell (I had to look up ls...) but I managed and went through the steps and could put the old settings on the new system. LibreElec 9.2.6 is running. Don't know yet if it actually worked with the user file transfer, because the library is still empty. Now I am struggling to mount my network shares from the NAS, but maybe I am just too tired an miss something. There does not seem to be a wiki page on how to do that. I hope I'll manage tomorrow. Other than that the time zone is not yet set correctly and not time server installed, I'll figure that out eventually. But time to go to bed now around here.
Thanks again for your pointers