Unique System Number for Old License Keys 
As of SAP Web AS 6.10, a unique system number is automatically assigned when the license key is released.
There are two exceptions to this:
The SYSTEM-NR of the new license is "INITIAL_SYSTEM_IDX"
All old licenses have "INITIAL_SYSTEM_IDX".
However only one SID is ever affected. If there are licenses for several SIDs in the system, only the SID specified in the new license is relevant. The new field is displayed on the initial screen of Transaction SLICENSE..
You can still install old licenses that do not have a SYSTEM-NR. When you are asked to enter the details, leave the SYSTEM-NR empty (do not enter blank characters, just confirm with Enter) and enter the other data. The SYSTEM-NR receives the value "INITIAL_SYSTEM_IDX". The old license is then valid as normal.
When a system is upgraded, all old licenses in the system are also upgraded. In other words, SYSTEM-NR gets the value "INITIAL_SYSTEM_IDX". The table is updated when this is done. The following steps are performed:
Read out licenses
Update licenses, that is, assign them a SYSTEM-NR
Delete table
Recreate table
Insert licenses