Just a word of warning - I am up to attempt 10, and at least the progress bar has started this time. The flashing appears *very* flaky. First time, mine got to around 90% then bombed out. Repeated, between 80-90% twice. Tried rebooting, advanced reset cover. 3 times didn't even establish comms and get to the progress bar. Up to 8% now (it is *very* slow) - we'll see if 10 is the charm
If you haven't updated, I would probably wait - once you start to flash, the cover is inoperable until the flashing succeeds. This means that quitting the updater/rebooting/resetting will all result in a completely black, broken cover display. I have probably just been unlucky, and this attempt will work wonderfully, but just so you have been warned - expect the firmware update to fail many times before it (hopefully) works.
I've entered a support request on the new cosmo troubleshooting page, so hopefully we might get some tips and tricks to getting it working more reliably.
PS. as a firmware engineer who flashes devices dozens of times a day, I understand there can be problems, but these seem a bit excessive.
EDIT: Yay - got to 100%, but still no cigar. "Error updating cover display!". Oh well, I should get better battery life now, and my smartwatch will do everything the cover did. Pity.