You already cannot map data automatically from one SAP instance to another, so forget auto migration. What usually happebs is, to keep the legacy system around for auditing purposes, and the live business happens in the new one. With cut-off dates per function and or department. With manually developed interfaces, and all the crap that comes with those, during the migartion period.

An nothing of this has anything to do with SAP, and everything with ERPs and the messy reality of businesses.