(thanks, Ian!), the SSN (system serial number) located in the NVRAM of the brick's L1 controller has to be set to L0000000 for the brick to obtain a new SSN from the neighbouring bricks.
SGI doc #108-0240-002 wrote:
When an R brick needs to be added to a system or moved to a different system, SGI
authorized personnel can clear the SSN that is stored in the L1 controller’s NVRAM.
To clear the SSN, SGI authorized personnel use a temporary authenticator generation
software program, which is located on a secure Web server that can be accessed via
WebSAFE (use your SGI login and password). Using the brick serial number, this software
generates an authenticator (four alphanumeric fields [total of 18 characters]) that is based
on the brick public key and the current date and time.
The SGI authorized personnel input this authenticator into the L1 controller
validation/serial number change software. The L1 controller software compares the
authenticator to the brick public key. When the authenticator matches the brick public key
and it has not expired, the L1 controller software clears the brick SSN. When the
authenticator does not match the public key or if the authenticator has expired, the L1
controller software does not change the SSN.
However, according to the same document, the L1 controller's NVRAM chip is socketed, so
the SSN is stored in cleartext, it could be rewritten without much trouble, hypothetically speaking.