(I've deliberately ignored the non-Nokia, non-Microsoft devices in the diagram below, for clarity. See the text below for some comment on other models.)
The diagram (hopefully) clearly shows the evolution of Windows and where the different Lumias sit - so, for example, the Lumia 930 started off with Windows Phone 8.1 (and is in that circle), got officially upgraded to Windows 10 Mobile 'Threshold' (ditto that middle circle) and is compatible with the next major code branch of Windows 10 Mobile, 'Redstone' (so it's in that circle as well, in fact in the intersection of the three!) Or, to take a newer example, the Lumia 650 started life on 'Threshold' and thus sits outside the 8.1 circle, while the Lumia 1020 remains on 8.1 and outside either of the Windows 10 circles.
Don't worry too much if you can't get your head around all this, Microsoft has it all worked out at their end.
One important takeaway is to understand the significance of the 'branches', these two codewords, 'Threshold' and 'Redstone'. They're both Windows 10 (Mobile), but they reflect significant milestones and the way the code for the OS is developed. You see, given the complexities of any modern operating system, no set of source code will ever be 100% bug-free, but the idea is to get as close to this as possible. So, at some point, the feature set and APIs for a given OS build are locked down and then only specific fixes relating to stability are allowed. This happened first at Windows 10 build 10586, on both desktop and mobile, which is why production devices and upgrades are still on build 10586.something. 10586 is codenamed Threshold and first appeared in the mobile world in November last year.
In parallel with this, from the moment Threshold was locked down in terms of capabilities, Microsoft's engineers were continuing to work on the source code, improving features and functions with more freedom - and it's this work that is codenamed 'Redstone'. The big jump in build numbers (into the 14,000 region) was to keep in step with work on the desktop, by the way. Redstone hasn't yet been locked down to a particular major build number (we're currently up to 14295), but I'd expect this to happen in the next couple of months, and then it'll be a few more months of stability fixes and minor build numbers (the bit after the decimal point) before we get to the Redstone over-the-air updates heading out to devices in the yellow circle above. So late summer-ish.
At which point the next codename should be known, for the next major development branch. And so on.
I've included a panel for Windows Phone 7.x for completeness, though it's binary-incompatible, being based on an entirely different OS architecture. It's worth noting that everything to the right of it is compatible though, evidenced by the number of Lumias which have made the trip from 8.1 through to Redstone Insider builds - and possibly beyond. Even to this day anyone can hop their (e.g.) Lumia 925 on the Insiders programme (just install the 'Windows Insider' utility from the Store, etc.) and enjoy Windows 10 Mobile in its Threshold builds - your experience won't be completely smooth anymore, which is why it's not officially recommended by Microsoft, but if you're happy with the odd bit of lag then you can rock Threshold on your three year old phone.
Apologies to anyone with a non-Nokia/Microsoft Windows device. For completeness, the BLU Win HD w510u, BLU Win HD LTE x150q, BLU Win Jr (1GB version) and MCJ Madosma Q501 are all approved for the Windows 10 Mobile (Threshold) upgrade, plus anything that started life on Windows 10 Mobile (e.g. Alcatel Fierce XL) is obviously going to be provisioned. Anything else that started with Windows Phone 8.1, including the HTC One (M8), HTC 8S, HTC 8X, Samsung ATIV S, Yezz Billy 4 and Yezz Billy 4.7, is destined to stay on 8.1.
The next big question will be which of the older-yet-Windows 10-approved devices will also be approved to get the official upgrade to the Redstone builds when they become mature and available. My guess is: almost all of them. The Lumia 635 (1GB) is currently excluded and I suspect that we may even see the other budget models join it to stay in 'Threshold' land, but the 640 upwards should be good to go for at least another year of major OS upgrades.