The amount of downtime when you change the processor frequency is the same time it takes when you change pools. It basically just does a reset, definitely doesn't take 3 hours.
You are totally right NeuralChris - it only takes the 5 minutes or so that a reboot takes - yeah. So I tried several different frequencies and it did not change the final frequency at all. The automatic frequency average was around 630, so I tried 650 as one of my attempts. The only thing I saw is that the fan speed maxed itself out (4080/6000). The frequency average / hash rate remained mostly the same (it bounces around a little anyway of course). So I can confirm that the hidden minerAdvanced.cgi option does not change the hash rate (and fortunately does not break the system). It is possible that it might be able to be used to change the fan speed though in case anyone needs that.
i been trying to figure out how change the fan speed because i got one S9 that works at 80* all the time the fans never go above 3000rpm. they stay the same even when i change the frequency.