Pretty easy but always good to be remembered: How to convert an ext2 to an ext3 partition. I had to do this on the /boot partition, so this can be done live as the partition needs to be unmounted.

The 'convert' process is just an activation of journaling, which is the difference from ext2 to ext3:

# umount /boot
# tune2fs -j /dev/sda1

Then /etc/fstab needs to be updated that /boot is now an ext3 partition (for the next boot process):

# cat /etc/fstab | grep boot
/dev/sda1            /boot                ext3       acl,user_xattr        1 2

Once mounted again, the file system type can be checked with df:

# mount /dev/sda1 /boot
# df -T
Filesystem    Type   1K-blocks      Used Available Use% Mounted on
/dev/sda1     ext3       93307     46284     42206  53% /boot