Hi team, is it safe to migrate tables with partial upsert or full upser feature from one group of servers to another group of servers without data loss?

Upsert should support rebalance, yes. cc: @Jackie

Yes, you may rebalance it using no downtime mode

Thanks. But if replica is 1, is it possible to rebalance with downtime mode?

Yes, you may use downtime mode to rebalance it