Changing MySQL Master in Replication: with or without MASTER_LOG?

Let's say I have server A,B,C. Both B and C have Master A A - Master B - Slave C - Slave

If I am going to change C's Master to Master B, can I change it immediately with query without Master Log information (edit: and also without dumping and importing the database)?

CHANGE MASTER TO MASTER_HOST='1.1.1.1', MASTER_USER='rep', MASTER_PASSWORD='password';

Or do I still need the Master Log Information ?

CHANGE MASTER TO MASTER_HOST='1.1.1.1', MASTER_USER='rep', MASTER_PASSWORD='password', MASTER_LOG_FILE='mysql-bin.000100', MASTER_LOG_POS=100;

If I need to Master Log information when switching Master, the MASTER_LOG_POS is running fast. How can I ensure that the number is not running? If I enter the wrong MASTER LOG information, what will happen to my database?

Thank you for reading.