From the Microsoft blog post announcing the new build (and see my highlighted phrase):

We’re rolling out Windows 10 Mobile Insider Preview Build 10549 right now to Windows Insiders on the Fast ring. However, this build comes with a caveat. We discovered a bug late last night that caused phones upgrading from Build 10536 to get stuck at a black screen after this new build is installed. Rather than making you wait a little bit longer for us to fix this bug – we decided to release this build as a single update from Windows Phone 8.1. Current Insiders that would like to try out Build 10549 should:

• Go back to Windows 8.1 first using the Windows Device Recovery Tool.
• Once you’re on Windows Phone 8.1, install the Windows Insider app, choose the Fast ring, and reboot your phone.
• After that – this build should download and install on your phone.

If you choose to remain on Build 10536 – that’s fine too. You will not receive today’s build but we’re working to ensure you’ll be able to receive the next one. We know having to go back to Windows Phone 8.1 to get this new build is a bit of work. By doing so you are also helping us test the upgrade path that millions of Windows Phone 8.1 customers will take to upgrade to Windows 10 Mobile, so we appreciate the adventurous Insiders who choose to go this route and give us feedback.

Here’s what’s new in this build:

Improvements to Cortana: Cortana now works for Insiders in Japan and in English for Insiders in Australia and Canada. For more information on new languages in Cortana – see my post here.

Messaging app improvements: The Messaging app now has a text box that can be expanded as you enter text. Previously, the text box had 2-3 lines which made it really hard to see what you were typing. As I mentioned yesterday with Build 10565 for PC, an upcoming build for mobile will include the integrated Skype capabilities for you to preview.

Diverse emoji: With this build, we are adding support for Unicode’s diversity emoji’s. Consistent with Unicode’s vision, we’re excited to offer people all over the world with emoji that reflect more human diversity. We added this capability to the PC with Build 10547 and now we’re extending to mobile. Check out our new diverse emoji and let us know what you think.

An update for Lumia 1020 users: Lumia Camera is no longer uninstalled when you update to Windows Mobile 10, so you can continue using it when you update your phone from Windows Phone 8.1. You should use Lumia Camera in order to take advantage of all of the Lumia 1020’s great camera capabilities. Lumia Camera is not yet available in the Store for Windows 10 devices so you will need to use the Windows Phone Recovery Tool to return to Windows Phone 8.1 in order to get the app back.

So there we go. Anyone wanting to stay on the cutting edge (like, err… me) needs to go right back to Windows Phone 8.1 and then forwards again. Oh well. It’s good for the soul. Apparently. Plus it helps Microsoft test the upgrade process that normobs will have to go through from 8.1, so this is you and I being public spirited.

It’s not yet clear whether one should set up as a new phone or restore a backup. Obviously I’m going to try the latter first. Comments welcome if you do this too! I do have multiple devices available though, so I’ll experiment and report back. It’ll take a few hours if not tens of hours….!

Initial thoughts are that there’s an extended period of ‘black screen’ before anything happens after this build is installed. So watch for that. You might need to nudge the phone with a power button press or two! In addition, as usual, allow plenty of time and battery power for up to 30 application installs through the Store client after the initial boot with the new build of Windows 10 Mobile.

UPDATE: There are so many glitches and things that just don’t work after the reverting and upgrading. I’m now in the process of hard resetting my Lumia 830 with 10549 on board and will report back! Confirmed. a full hard reset is also needed – full article and assessment coming soon.

There are some words of warning about Cortana and battery life, by the way, from Gabe Aul, who runs the show over at Redmond:

@NN2710 There are a couple of fixes in this build, but we have a big one coming still. Cortana BG usage can be super high in some cases.

@NN2710 You can turn off Cortana as a temporary workaround.

Gabriel Aul (@GabeAul) October 14, 2015

PS. While you’re waiting and downloading, make sure you’ve also read my ‘Windows 10 Mobile a step-change, but ultimately needed on many levels‘, plus my recent benchmarking features.