I ran the urpmi.addmedia command and followed the instructions in "Upgrading from Mageia 7",
running "urpmi --auto-update --auto --force --download-all" several times.
On rebooting I found that I could not boot into the backup root partition. This still has Mageia 7: a copy of the main root partition which I could boot into before updating.

I could still boot into Windows.
On booting into Mageia 8 I get the message "Building nvidia-current driver..." and then a blank screen.
I can bring up a shell with Shift-Alt-F2, but Shift-Alt-F7 gives a blank screen with a cursor.
The bios error messages (which seem harmless and which I always get on booting) switch to a smaller font: which I seem to remember happened when I first set up the machine and was using the wrong graphics driver.
On rebooting the message "Building nvidia-current driver..." still reappears and I still get a blank screen.