* OK, apart from an extreme hack, which I'm err... 'investigating'!
From the MSPU article:
Earlier this year, all Windows Phone 8.1 Lumias and a select few other phones could get Windows 10 Mobile, provided they signed up for the Windows Insider Program and downloaded the new bits from one of the available rings.
Even after Microsoft’s Windows 10 Update rolled out officially and the firm blocked older devices from accessing the fast ring, old users ould still make use of the Release Preview ring to hop right to Windows 10 Mobile and take advantage of the new apps and benefits that come with the new OS.
Now reports are rolling in that new users can no longer take advantage of the Release Preview work around. Rather, any user who’s stuck on 8.1 will have seen their last update, while users who are on Windows 10 November Update will have to make do with that os version warts and all. They can not roll back to 8.1 without being stuck there, nor can they progress on to Windows 10 Mobile Anniversary Update, leaving them trapped between a rock and a hard place.
'Stuck between the devil and the deep blue sea' is a phrase that comes to mind for owners of the Lumia 520, 620, 720, 820, 920, 925 and 1020 (plus variants and some other models from the same era). They've had 18 months to try the jump to Windows 10 Mobile and I'm guessing if they're still on Windows Phone 8.1 then they've made their choice - but very little new is coming out for WP 8.1 and there are certainly no OS updates.
On the other hand, anyone who has upgraded an older device to Windows 10 Mobile 'Threshold' via the Release Preview Insiders ring (the one that's just moved on and no longer available) is now also rather stuck, in that only a handful of minor bug fixes are expected to come out for Threshold and these won't come to older 'Release-Preview-upgraded' devices at all, since they're not provisioned anymore.
Almost all Microsoft approved 'Windows 10 Mobile compatible' phones are upgradable to the Redstone branch, and automatically so, i.e. there's no involved Upgrade Advisor or similar - so almost every device will in fact be on Redstone very shortly (a few months at most). So, after July 2016, Threshold, the 10586.xxx branch of Windows 10 Mobile, will be effectively frozen by Microsoft - there's no need for updates because almost every W10M-compatible device will be on a wholly separate branch. So no more security updates, no more bug fixes, and so on. Still, at least users are on a 2016 OS rather than a 2014 OS (Windows Phone 8.1). And there's always the possibility to revert to 8.1 via the Device Recovery Tool if the user feels that Windows 10 Mobile Threshold isn't fast enough or beneficial enough....
An interesting point in the Windows 10 Mobile timeline then - and an elbow in the ribs for me to update some of my charts!