The links in the original post died. Luckily, a helpful friend named Guillermo shot me an email with the updated firmwares you'll need.
First update to this one: http://dl.ubnt-ut.com/usw/US.bcm5341x.develop.10223.190407.1857-uboot.bin
Then update to this one: http://dl.ui.com/unifi/firmware/USXG/18.104.22.16898/US.bcm5341x_5.43.35+12698.210326.0445.bin
Keep in mind I have not personally tested the above path, so YMMV.
Original post is below for posterity. Thanks Guillermo!
A while back we picked up a couple Ubiquiti US-16-XG switches for $299 each during their beta period. These are 10G switches, 12 SFP+’s and 4 RJ45’s, at a pretty crazy price. But then I got busy and didn’t mess with one until now.
Since some time has passed, the US-16-XG is now out of beta. And nothing has changed in hardware, so upgrade the firmware and you got the production product for half off.
Easier said than done.
Unifi didn’t give me the Upgrade option, and I kept getting Invalid Firmware and Invalid Version errors when doing it manually.
Solution – I had to put transitional firmware on it:
Using the SSH method:
Only after I did that could I update the firmware properly via Unifi.
If you’ve already adopted the switch in Unifi, the SSH creds are the same as you use for Unifi. If not adopted, it’s the default ubnt/ubnt.
I’m posting this solely because it took me hours to track down the thing about the transitional firmware. Hopefully you got here after having wasted less time.
And hey, now you have a $599 switch that you got for $299!