FWIW, i know this thread is old...
but there are many ways to change the serial# including apple's blank board serializer and more.
the problem being, most of these tools only change the ASCII string in the EEPROM.
the serial is stored in plain text, but there's also a hash calculated from that serial.
if you change the serial, system profiler will display the new serial, but AHT (apple hardware test) will report unknown serial, and apple servers will still identify the mac by its original serial.
it's totally possible to monkey with the serial, over and over again... although there's no real benefit