Revised proposal: for MVP, a method exists for a community to import data from its corresponding SE community in a license-compliant way. It is up to the community to decide whether and how to use this; a community might decide to start fresh, import selectively, or import everything and continue operations here. A manual, one-time operation meets the need.
License-compliant could mean importing only data that predates the latest questionable license change, or could mean applying different licenses to pieces of content based on last-write date on SE.