Mysql privileges and replication
You are over-thinking this.  Stopping the slave doesn't cause you to lose your binlog position, and changing the replication username and password doesn't, either, as long as that is all you change.

On the slave:

mysql&gt; STOP SLAVE;


This won't stop in the middle of a binlog read.

Create the new account in the master, then reconfigure the slave to use them:

mysql&gt; CHANGE MASTER TO MASTER_USER='foo', MASTER_PASSWORD='bar';
mysql&gt; START SLAVE;


Unless you change other parameters with CHANGE MASTER TO, the slave will reconnect with the new credentials and resume where it left off.  You do not need to specify host, port, file, position, or anything else, and you should not specify them, unless they are changing too... in which case, yes, you'll need to restart with new coordinates.

https://dev.mysql.com/doc/refman/5.6/en/change-master-to.html