Updating mine now, currently looking at the new splash screen on repeat so I hope it's doing something! Just so it shows up in searches for other people that might came across a weird issue I had, I was stuck having unlocked the bootloader and rebooted ready to go, but just kept getting this weird error:
fastboot flash bootloader bootloader-hammerhead-hhz12k.img
target reported max download size of 1073741824 bytes
sending 'bootloader' (3120 KB)...
FAILED (data transfer failure (Unknown error))
finished. total time: 5.036s
Using a decent USB cable fixed the issue even though all the previous steps had worked with my normal cable.