In pictures: Comparing Windows Phone 8.1 and Windows 10 Mobile, side by side

The day has finally come. Microsoft originally planned to begin upgrading some Windows Phone 8.1 devices to its newest mobile OS at the end of last year, but following a series of delays, it finally announced the beginning of its Windows 10 Mobile rollout today for (some of) those older devices. You can find out more about how to upgrade in our guide here.

If previous rollouts are anything to go by, the process is unlikely to be a speedy one - carriers and manufacturers will still have to approve the upgrade before it makes its way to all eligible handsets, and some devices won't be updated at all.

So while the upgrade has begun today, the long wait for its arrival on your device may not be over just yet, although if you get tired of waiting, you can always install an Insider Preview of the new OS in the meantime. That said, the Insider Program isn't for everyone - and if you're not particularly confident (or competent) in dealing with bugs and other issues, it's best avoided, especially on your main smartphone.

But if you're keen to get a glimpse of what Windows 10 Mobile has to offer, and how it compares with what's already installed on your device, we put together a gallery with dozens of images showing Windows Phone 8.1 alongside the new OS that will eventually replace it.

From the 'onboarding' process of setting up a device for the very first time from a clean installation (which you won't have to do if you're simply upgrading), to some of the key settings that you'll probably want to tinker with after the new OS is installed, and a look through some of the key pre-installed apps on both the old and new operating systems, the side-by-side images should give you a broad sense of how Windows Phone 8.1 and Windows 10 Mobile compare.

Of course, nothing quite compares to actually having the OS in your hand - but while you're waiting for it to arrive, be sure to check out the gallery below.

Screenshots have been captured from Windows Phone 8.1 Update 1, as this is the most widely used version of the OS installed on the greatest number of devices (Update 2 was not widely released). Windows 10 Mobile screenshots were captured from build 10586.107, but any UX changes between this and the latest 10586.164 release are minor. All screenshots used default 'out of the box' settings from a clean installation of each OS.