Hi Frank and Stephane,
thanks for yor replies.
Stephane wroteIf you are familiar with Linux and that your LocalStorage card is ext4 formatted then you can put the new 0.4 files without having to reformat the card.
Unfortunately, my third partition on the SD is, for whatsoever reason, exFAT. However, as far as I can see /boot is identical, given the same kernel version, for 0.3 and 0.4. Thus, cloning the new 0.4 /root by tar cf - | (cd /mnt/root; tar xf - ) should do the job on a running 0.4 with the cwd /root and the root partition of the 0.3 SD to be updated mounted on /mnt/root? Any objections Frank?
Best,
RRPi3