Hi Jens,
For creating an Technical System for a Third Party System (in your case legacy CRM System) in SLD, it asks for 'hostname' as a mandatory thing. But there is no harm/impact if you pass anything in the 'hostname'.
Hence if 'hostname' or any technical detail if changes in future, it won't create any problems. So that won't affect your design/configuration of the systems as well as your process flow.
Regards,
Amit Patil