This is something that is not explicitly decribed in any help page or F1 help, but may make you wonder.
To assign a system ID to a source system, you need to go to the following menu option in RSA1:
After clicking on it, you will enter the “Maintenance of Source System IDs” screen. You will notice that some IDs are not changeable, like the system Q9 in the picture below.
The reason why this field is disabled is that you already have master data loaded for this system. The 0SOURSYSTEM’s SID table from the next screen-shot confirms it.
On the other hand, the following pictures show a BW system with no master data loaded for any of its sourcesystems. The first screen-shot shows all soursystems IDs are editable, and the second shows that the SID table for the InfoObject 0SOURSYSTEM doesn’t have anydata.
Note that the SIDs Table’s key field is the SourceSystem ID. You cannot change a key field as other tables may have references to it. Therefore, to change the ID of a source system, you would need to first delete the data realted to the source system loaded to InfoProviders and then delete the master data.