MySQL : if server_id is not explicitly set, then get error `ERROR 1200 (HY000) at line 1: The server is not configured as slave;`
Two servers are not allowed to have the same server_id. Also, If you omit --server-id from a master, the default ID is 0, in which case a master refuses connections from all slaves, and a slave refuses to connect to a master.

For more information, see Section 15.1.1.2, “Setting the Replication Slave Configuration”.

Even if both servers have different server_id values, please make sure the new master has binary logging enabled.