It appears that the default boot partition is too small for F12 preupgrade:
Did anyone try to see if enough space is freed by following these steps? :
1. Leave only 1 kernel
2. tune2fs -r 0 /dev/xxx
3. remove /boot/efi and splash.xpm.gz
"preupgrade now requires at least 167MB" means more space could be needed,
and I'm unclear if the above is sufficient.