As I mentioned in the recent podcast, this is all just software at the end of the day and there's nothing impossible about Microsoft provisoning the older Lumias to work with the 'Fast ring' Redstone builds - but I sincerely doubt this will happen. If these 3 year old devices weren't deemed powerful enough to run Threshold then it's unlikely that the decision would be reversed for a newer, more complex version of the OS. It might happen, but don't hold your breath.
So what do you do if you really, really want to see how far you can travel down the rabbit hole with an old Lumia? Well, thanks to various 'how tos' online, there are ways to unlock an older phone, get access to its registry and replace the relevant strings to mimic a newer device that IS supported for the Redstone Insiders builds. However, having reviewed the various videos and guides, I should warn you that the steps needed are very non-trivial and will take a while to figure out and then perform. If you do proceed, make sure you allow several hours in all and whatever you do, don't do this on your main phone, since you will probably come a cropper at some point. Redstone is problematic enough at the moment (as of build 14322) without adding unsupported hardware into the mix as well.
But for the record, from this page on XDA Developers:
1) install windows insider from store
2) Download WP sdk tools 8.0 lite :source zippyshare and click on "install.bat " click "Yes" for every prompt
Goto ("C:\Program Files (x86)\Microsoft SDKs\Windows Phone\v8.0\Tools\XAP Deployment") and open "Xap Deployment.exe"
3 ) Deploy @vcfan Lumia registry editor vcReg_1.5
[ Go to vcreg-->(. . .)-->templates-->check mark , live interop and restore Ndtksvc and click on apply
try to deploy custom pfd 6 with same "xap Deployment.exe",if its not successful then
3 ) Download [B]WP sdk tools 8.1 lite source Zippyshare and extract it,click on "InstallWP81SDKToolsLite.bat" accept all prompts .
goto "C:\Program Files (x86)\Microsoft SDKs\Windows Phone\v8.1\Tools\AppDeploy" and open " AppDeploy.exe" .
download customPFD (@Pasquiindustry) and deploy it.
move to HKLM-->SYSTEM-->Platform-->DeviceTargetingInfo then
take the screenshots of your device original values and move to SDcard,you can restore same values anytime
a)change " PhoneManufacturer " = MicrosoftMDG
b)change " PhoneManufacturerModelName " = RM-1085_11302 (for Single SIM devices) // *** For all dual sim devices try = RM-1116_11258 ***
4) change PhoneModelName = Lumia 950 XL (single sim) or Lumia 950 XL Dual SIM (for dual)
5)4) change PhoneHardwareVariant = RM-1085
Typical XDA geek-level instructions then, though the links are very helpful. If you want to get a sense for what most of this looks like then there are video versions of the above online, such as the one embedded below (though it misses out the last step, annoyingly):
If the video and instructions haven't put you off already then you're doing better than me. I'm up for a little tweaking, but the procedure here seems overly laborious for very little gain. After all, is Redstone that much better than Threshold that you're willing to serious compromise the stability of your phone and go through all the hassles of updating and then, in all probability, reverting back soon afterwards, knowing that you'll never be sure whether problems you experience are because of bugs in the OS or because of the unspported hardware you happen to be using?
Comments welcome anyway, I'm sure someone here has a few hours on their hands to get playing with all this on a sacrificial x20 device? Just don't blame us if it all goes pear-shaped!