Third time's the charm, unless they plan another release at some point.
The StarCraft II interface isn't perfect. Even though it is interesting and visually appealing, some tasks are unnecessarily difficult and space is not used in the most efficient way. To see what I mean, try to revert the multiplayer mode to Wings of Liberty, or, worse, find your Character Code. Blizzard released a new UI with Heart of the Swarm back in 2013, and they're doing a new one for the release of Legacy of the Void on November 10th. Note that my two examples probably won't be fixed in this update, they are just examples of UX issues.
While the update aligns with the new expansion, Blizzard will patch the UI for all content levels, including the free Starter Edition. This honestly makes sense, because it's easier to patch a title when all variations share a common core. Then again, not every company patches five-year-old titles like Blizzard does, so the back-catalog support is appreciated.
The most heartwarming change for fans, if pointless otherwise, is in the campaign selection screen. As the StarCraft II trilogy will be completed with Legacy of the Void, the interface aligns them as three episodes in the same style as the original StarCraft did.
On the functional side, the interface has been made more compact (which I alluded to earlier). This was caused by the new chat design, which is bigger yet less disruptive than it was in Heart of the Swarm. The column of buttons on the side are now a top bar, which expands down for sub-menu items.
While there are several things that I don't mention, a final note for this post is that Arcade will now focus on open lobbies. Players can look for the specific game they want, but the initial screen will show lobbies that are waiting to fill. The hope seems to be that players waiting for a game will spend less time. This raises two questions. First, Arcade games tend to have a steep learning curve, so I wonder if this feature will slump off after people try a few rounds before realizing that they should stick with a handful of games. Second, I wonder what this means for player numbers in general — this sounds like a feature that is added during player declines, which Blizzard seems to hint is not occuring.
I'm not sure when the update will land, but it will probably be around the launch of Legacy of the Void on November 10th.