Problem reseting serial number after system board replacemen

[cmra31p]

I have an A31p. I used the ThinkPad Harware Mainenance Diskette Version 1.74 to reset my serial number. I chose item "20" as instructed and entered my machine type and serial number using the prefix "1S". It then stated that the information was already entered and when I restart the serial number hasn't changed. Should I choose item "40" instead? Everyting I see says to use item "20" but item "40" on the program indicates it is for the systrem board.