For some reason, I was not able to get the Google Chrome trick to work.  I change the value, click save, and it comes right back up at 500mhz.

However, what did work was to save the configuration file from my first batch Z9, and upload it to the new Z9.  Just select what frequency you want before saving the config file on the first batch machine, then upload it to the second batch machine.  The second batch machine will mine at the frequency selected.

First batch machine has been running at 750mhz since day one.   Second batch machine would not hash at 750, so I'm hashing now at 675 with about 15Khash.