Hi, I tested the firmware on the S9 13.5 T model and S9i 14 T model and it's great !
I have around 200 old S9 with fixed frequency of 550 MHz can I use the Braiins - OS firmware on the fixed frequency models as well ?
As far as i know, the only possible limitation could be the controller itself:
What Antminer S9 flavors are supported?
All S9 variants (S9, S9i, S9j) are supported, however, the amount of possible variations is rather high. Initially, for this reason we recommend testing our SD card images first. Once you are confident that the firmware operates according to your expectations, you can apply factory transitional firmware images.
So as suggested, try booting one unit with the sd card first. If it doesn't work, you remove the card and move the jumper back and its like nothing ever happened. No worries.