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I just tried this, and if you turn it off in control center the watch stays connected via bluetooth even though it looks like it's disabled. Screenshots attached for proof.

If you turn off bluetooth in Settings with the toggle switch (which actually turns it off), when you go back to the Watch app it says you must have Bluetooth on.

I always do that for my Watch and it's never failed me. I don't get any popup asking me to enable Bluetooth.
Steps I take:

- Update is released
- Turn off BT completely from settings (not from CC as it just temporarily disconnects device, allowing Watch to reconnect)
- Head over to watch app, look for update
- Even if watch had already been found and it was downloading (most likely via BT), UI will "refresh" and task will resume, this time via WiFi I presume (as there are no other connections available, having completely disabled BT)
- Update will be performed much faster (still rather slow, but much faster than BT)
 
This update puts the Watch in a battery hog-mode. It's like a day and recharge ...
 
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