Set the anonymous session store to whatever was decided in T402850: Decide on anonymous session backend. By this point, we should have verified via T402808: Deploy separate anonymous session backend to Wikimedia production, in log-only mode that the split backend works as expected, so this is low-risk (unless there's risk on the infrastructure side). We can probably just roll to group 0/1/2 in three successive days.
When a wiki switches, all current anonymous sessions are lost. That should be fine, a few people will have to restart login/signup or resubmit their edits but there will be no data loss.