I got my update few days ago, luckily I have installed some Market app before. Able to switch to the Market app after system upgrade and install additional launcher.
Now when press the Home button, it will prompt to select launcher.
FYI, I have installed:

1. Shafa Market & Shafa Launcher http://www.shafa.com/
2. DangBei Market & DangBei Launcher http://www.dangbei.com/
Both are Chinese, for those who cannot read chinese, maybe you can try others:
The 7 Best Android TV Launcher Apps
Best Android TV Launchers for your TV Box for 2019
Top 10 launchers for your Android TV Box

If you have installed some app before, chances are it may already run in the background. If you see the icon at the left bottom corner, try to press menu button, and you may switch to the app.
If you are not able to switch to any app, try download launcher apk into USB drive, plug into the TVBox, if lucky, you may prompt to open the USB and install the apk, good luck!

Advice for those who hasn't upgraded yet, install Market & Launcher ASAP.

This post has been edited by YiQi: Oct 9 2019, 01:07 PM

Hmm...yes, everything will not appear in the new UnifiTV UI.
But you can try the "Home" button, then a popup will let you choose which launcher to run.

You have switched your Unifi package, maybe you need to doublecheck your UnifiTV ID & Password, probably changed.

Yes! Me too, but alternatively, install Smart Yourtube TV works well.

Yes! I upgraded each version.
It works, most of the time, just crashed occasionally (especially last version).

Not really. My account has been migrated, phone using latest PlayTV@Unifi, home still using old white box (VLAN) without any issue.

Suspect new Unifi IP range that not regconized by BBC.
Maybe can try restart your internet router to get diff IP (diff range).