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.