I’m helping an elderly lady to get an Apple Watch. It would give her a very convenient way to monitor her heart rate. There is no budget to buy a new watch or a new iPhone. I am confused which watchOS version will support her iPhone.
iPhone 7plus with iOS 15.x, the highest it can take.
She bought a used Series 6 Watch, it came with watchOS 11.5 on it
Series 6 originally had watchOS 7, this would have been compatible with iOS 15, but obviously the previous owner upgraded watchOS at some point. Fair enough.
Apple support had told me that we could send a watch in to have it downgraded.
Once the watch had arrived here, I called Apple support again to ask about shipping details for the downgrade. To my surprise the agent said it definitely can’t be done at all. He checked with supervisors and came back with the same answer.
I’m not sure that A) this is correct and B) what else I could read from the aforementioned page to find out which used watch model would definitely work with the given iPhone.
The Series 5 and SE 1 were able to install watchOS up to version 10.6.1, so anyone who had upgraded would require a phone that can run iOS 17 or higher. So I’d say stay away from this model.
The last model of watch that went up to only watchOS 8, which works with iOS 15, is the Model 3, which would be quite old by now.
Honestly you may be better off upgrading her phone to an iPhone SE 2 if she needs to have a phone with a home button, if you can find one used.
I did a bit of digging, and it appears that the person you spoke to was correct. It can’t be done. But the possibility is tantilizingly close.
Since you have a series-6 watch, you still have the secret diagnostic port, hidden behind one of the strap-connectors. Using that port (with a compatible adapter to connect a Lightning cable to it), you can put the watch into DFU mode, connect it to a Mac and install a firmware image file.
This would sound like great news, but…
I can’t find any place to download IPSW firmware images for a Series 6 watch. I did find sites (like this one) that share links to Apple’s CDN servers, but all of those sites stop at images for Series 5 watches.
I would never consider downloading an image from any server other than Apple’s. There’s too much risk that it could be corrupt or infected with malware.
Even if you find a source for a Series 6 image, Apple digitally signs their images (like they do for iPhones and iPads), and they usually stop signing old images. Which means the only signed images are probably going to be the watchOS 11.5 you’ve already got, and older images will refuse to install.
As @ddmiller wrote, your best option is probably going to be to upgrade the phone. Look for a used/refurb model running iOS 18 or later in order to be compatible with the watch.