With the huge caveat that the lists and information here contain a modicum of guesswork, here's the table as at the start of November 2015:
|Update type/date for
Windows 10 Mobile
|User data and
|Arriving with it on-board:||n/a||Lumia 550, 950 , 950 XL|
|Over the air update
starting end December 2015
|Yes||Lumia 430, 435, 532, 535, 540, 640, 640 XL, 730/735, 830, 930, HTC One (M8)
|Over the air update
starting February 2016
|Yes||Lumia 520, 525, 620, 625, 630, 635, 720, 820, 920, 925, 1020, 1320, 1520, Icon, HTC 8X|
|Update via PC utility,
starting March 2016
|Backup/restore needed||Lumia 530|
|Likely to stay on
Windows Phone 8.1*
|n/a||HTC 8S, Samsung ATIV S, Yezz Billy 4, Yezz Billy 4.7|
* any phones not mentioned (barring typos from me and the odd 'dual SIM' variant) are likely to be from lesser manufacturers and included in the bottom row. I'll update this table if this changes for a particular company. The bottom row may also include some network-branded Lumias where the carrier is either no longer around or not playing ball with Microsoft(!)
Of course, in a sense it doesn't matter to anyone reading this article - you'll already know about the 'Windows Insider' programme and application and can jump on the new version as soon as you like - as of Build 10581 it's ready for more mainstream use. But hopefully the table is accurate enough and complete enough that it can be used as a reference for other users, for whom the phone is just a tool (rather than an object of geek passion!)
An obvious question is "Why is it taking so long, why not upgrade all phones at once?" There are two reasons, the biggest being that Windows 10 Mobile is only now just being finished off - there are still oddities and glitches being ironed out, even for top end phones with the latest chipsets. There's a good chance that the Lumia 950, 950 XL and 550 will launch with an OS that's largely bug free (and with numerous Store updates to follow through December to polish things up further), but I'd go no further than that.
The next stage is much harder - to send out an update that will upgrade users firmly established in Windows Phone 8.1, with all applications, data and settings, and to see them upgraded with minimal disruption and without mass calls to helplines and general support. On the Insiders Programme, I've said many times that Windows 10 Mobile is much smoother and solid when a hard reset has been done at some point in the flashing process - Microsoft doesn't have that luxury for 'normobs', who won't have a good handle on where all their data is and how to recover if things suddenly look empty or different. Microsoft has to get it right first time.
Which means masses of testing and patching, rinse and repeat, and most of this will happen in December 2015, finishing off with a limited rollout of OTA (Over The Air) updates being made available to the phones listed above towards the end of the month. Heck, it could even slip into January 2016, I wouldn't be surprised. Based on how well all this is going and how many issues people have, the device pool then gets a lot wider for the next wave, at least a month or two down the line.
Again, I'd stress that the usual frustration shown around these parts when OS updates aren't forthcoming is completely avoided by the existence of the Insiders Programme, which cuts across all geographical and network boundaries. Anyone who really wants Windows 10 Mobile 'now' can get it by simply installing a tiny utility and waiting while a few cogs 'spin'...
The second reason for the delayed update rollout is, as ever, that networks around the world still need to do some internal testing with every variant with the new firmware, and give their approval for each and every variant. Once approved and updated, they shouldn't need to get involved again for a while though, given how much (right down to application runtimes and internal layers) can be updated via the Store these days, pushed to every Windows 10 user directly.
It's unlikely that we'll see networks hold up Windows 10 Mobile unduly. The company has already said:
“Microsoft is working closely with mobile operators to leverage their testing and our flighting to meet and exceed current quality bars. We will use their input but will decide when to send the updates out, based on input from Mobile Operators and our Windows Insiders.
You'll note that the bottom two rows of the table address special cases. In some rare cases, phones only have 4GB of internal storage, making it almost impossible to upgrade the OS 'in place'. As it is, even with other phones, a user who has nearly filled their internal disk and microSD card will be asked to clear space (which will almost certainly put a lot of people off even going for Windows 10 Mobile in the first place) - with a 4GB device, even a newly factory reset phone has issues performing the upgrade in place, so a 'Recovery Tool' solution will have to be provided, in conjunction with instructions for backing up to OneDrive and then restoring, I suspect.
Finally, there will be handsets which don't even get offered Windows 10 Mobile, usually because the manufacturers haven't got (or don't want to spare) the resources to do all the testing needed and provide support. This is unlikely to be a big deal for many users, since the handsets concerned are either budget devices or, simply, quite old now, and anyone using them will either be happy with current operation or will have moved on, in which case the phone is a plaything or media player for a child, for example.
Overall, the road map for official updates is at least partially clear though - we'll report back on any major OTA rollout events of significance, but in most cases anyone who cares enough about the whole topic will already be living, as we are, in Windows 10 Mobile most of the time already.