We moved to a new DB as a clean slate, took fresh orders, and then merged the old ones in as they became available. Was able to do that because all primary keys were uuids and the order history API was being served somewhere else.
This microservice didn't have them, but even they're all keyed by UUID, so it's possible to start a clean slate pretty fast. Filling in existing users later should also be easy, the problems will occur if and when users try to sign up again with the same key (email / FBid etc).
