Source System Replacement - Steps in BW4

Dear SAP experts,

We have BW Q system which is connected to S4 Q system .

Let's say currently S4 Q02 700 is connecting to BW4 Q as source system.

Now we need to retire Q02 700 system and use another Q01 500 as BW4 Q source system. Let's say S4 Q01 500 system client will replace Q02 700 as new source system.

I would like to know about all steps we need to perform , pre-process activities , etc..in BW4 and S4 new and old client.

I'm dying to get your suggestions and thanks in advance.

Regards,

Guo Lei