Were you able to downgrade it at all, even temporarily, before the drivers updated it back to the newest? I mean, does it downgrade at all or does the “upgrader” just say “no need to upgrade, the module already has a newer firmware”?
If the downgrade was possible, I would be very happy & grateful if I also met that kind of a very nice person who could send me the older firmware. 
So I have a similar problem with 2 pcs of MC7455 but AFAIK they use the same firmware as EM7455. My application is freeBSD, or actually pfSense firewall package built on top of freeBSD. It worked nice, but I was curious to see if the upgrade to the newest SWI9X30C_02.24.05.06 brought any new functionality. At first I thought it worked ok so I also upgraded the other module too. Thereafter I met a serious problem and now I have two useless (from my point of view, non-functional) modules.
I had copypasted the result of ATI command before the upgrade, this is what I had before and it worked, so it’s also what I’m now looking for:
Revision: SWI9X30C_02.20.03.00 r6691 CARMD-EV-FRMWR2 2016/06/30 10:54:05
It’s essential the firmware would be a generic one, not any operator-spesific version.
Sierra’s website obviously offers only the newest one. Or is it available somewhere? I did find some legacy drivers etc but not the firmware.