[rsync]

"I've been trying the Windows 10 betas on a fairly high-end Lumia (1520)"

Do I read this to mean that older WP phones that ran WP7, etc., can then be reloaded by the end user to use other WP OSs, such as WP10 ?

That seems surprising to me for some reason. Did you have to root it, or the equivalent of rooting ?

What else could you load on a phone like this ?

[pavlov]

This is the only part of Windows 10 Mobile that Microsoft is doing right. Practically all Windows Phone 8 devices -- even the 89 euro ultra-cheapies like Lumia 520 -- can be upgraded to 10 without rooting or any other special tricks.

Currently it requires you to install a "Windows Insider" app, but the final version will presumably be available as a regular OTA upgrade just like 8.x updates.

[paulsnar]

You can't on WP7 or older devices because of kernel incompatibilities. But pretty much everything that can run WP8 can be upgraded to the official WP10 beta through OTA.