I reckon it'll be made available to 950/xl, 4s and x3.
I think there's a good chance MS wants to modernize the hardware standard somehow.
Reading between the lines, I think FCU will be a HUGE update, include cshell, Cortana skills, FDS, nitelite, the changes to the keyboard - but that MAY be it for existing phones. That however would bring the existing flagships up to parity with whatevers next for awhile.
Obviously new form factor "Andromeda" device will be released this year or next, and whatever MS and HP are cooking up together for the x3 refresh should be this year- and I have a feeling they'll be using those two devices as reference hardware from 2018 on.
Let's be honest 64 bit is overdue, so it might just be they want to switch to that. Whatever the reason, they discouraged whartonbrooks from making their phone saying they wouldn't support it in the FCU - that implies that FCU may be the last great update for us, or at least that they intend on slowly culling the list of supported devices so they can move onto a new build for newer hardware somehow. (it's possible I suppose they'll support the big four, for a few more updates after that yet, this is largely speculation!)
64 bit would make a lot of sense, to explain this, because QUALCOMM don't make 32 bit chips anymore. But we should IMO get the FCU for "the big four", it should be HUGE IMO, and past that ---shrug.
I don't think 650/550 or the others will _handle_ the changes. The changes to the UI alone may be too much for 1gb ram (Ie fluent design).
Some of those older phones were up to spec but were scrapped because they were hard to maintain compatibility with win10m for. Ie they were windows 8 era phones.
However it's likely IMO you'll be able to hack an install some way, just not guaranteed that people will like the result. I could be wrong there - you could maybe just switch FDS off, or remove transparency or visual effects, and then otherwise I don't see why not...
More shrugs?