S/4HANA (ERP, CRM, SRM, SCM, PLM co-deployment)

Hi Experts

I am a bit confused.

I know that with S/4 HANA the ERP, CRM PLM and SCM are co deployed in one system and now there is not need to different hardware.

Please explain if a customer want to migrate to S/4Hana and they already have CRM, SCM, PLM and ERP in different hardware. How this migration will be performed. How these will be migrated to merge into one system.