I had found it a while ago from another post on here:
Try this receipe:
The current System ID is 987654-32-0. Say you want to change this to 987654-32-1.
First halt the VSIM and enter into the bootloader prompt. Set the new serial ID there as shown.
SIMLOADER> set SYS_SERIAL_NUM="987654-32-1"
On booting if the system complains that it can't find it's disks then that's because software disk ownership is doing the right thing and keeping the disks allocated to the older system (old serial number). If so, ctrl-c into maintenance mode and re-assign the disks to the new ID.
I hope this response has been helpful to you.
At your service,
NetAppU Instructor and Independent Consultant
(P.S. I appreciate points for helpful or correct answers.)