change FAT16 partition UUID
mtools comes with an utility
mlabel which might do the job.
mlabel -N aaaa1111 -i /dev/sdb1 ::
Apart from that you might have to resort to a hex editor. The
dosfstools only lets you change the label using the
fatlabel command (which
mlabel does too, just without the volume id).
If you're willing to re-create the filesystem from scratch, the value can also be supplied via
mkfs.vfat -i VOLUME-ID.