Today I tried to upgrade a system which apparently hadn't been upgraded for ages: it still ran 42.3.

I did it he way I am used to: choose for a fresh install, and import the filesystem layout and mountpoints from the existing system.  (It still puzzles me why the default proposition from the installer always is to re-arrange the existing filesystems on disk, in a way that is destroying all data on it.. Why not at least keep the /home partition intact?).

In this process I get the warming

The system might not be able to boot:

And I'm given the choice to continue or not.

The message as such is somewhat ambiguous to me: is a partition of type BIOS Boot Partition needed for the new installation, and is it non-existing?
Or, the other way around, does it exist and should it be deleted for a successful install?

Moreover, the message doesn't give a clue as to what to do.

This warning slightly resembles another one, for which I asked for and got help for the 10th of October 2018 (https://forums.opensuse.org/showthread.php/533410-Warning-about-missing-bios_boot-labeled-partition-in-install-procedure); there I decided to continue, and installation went without problems.

Therefore, AND not knowing what the installer wanted,  I decided to continue.

But this time the installation itself went smoothly, but left me with an unbootable system: there is no valid partition found to boot from.

The partitions on the disk are:

/dev/sda1   4.00 GiB Swap  on Swap
/dev/sda2 50.00 GiB BtrFS  on /
/dev/sda3    0.86TiB XFS     on /home
/dev/sda4    1.00 GiB FAT    on /boot/efi

Why did the system boot on 42.3, and refuses it to boot from 15.2?

More important, what can I do to make the system boot? Preferably without touching /home...

Your help is greatly appreciated!

regards, Jogchum