If you specify the `MASTER_HOST` or `MASTER_PORT` option, the slave assumes
that the master server is different from before (even if the option
value is the same as its current value.) In this case, the old values
for the master binary log file name and position are considered no
longer applicable, so if you do not specify `MASTER_LOG_FILE` and
`MASTER_LOG_POS` in the statement, `MASTER_LOG_FILE=''` and `MASTER_LOG_POS=4`
are silently appended to it.
Do avoid the resetting of `MASTER_LOG_FILE` and `MASTER_LOG_POS`, we
generated the dump after setting the `MASTER_HOST`
https://dev.mysql.com/doc/refman/5.0/en/change-master-to.html