The update to version 2.0 ushered in some major changes to YouTube for Android TV. Many of them were superficial, but there were also useful things like being able to turn off autoplay. It also got a fair amount of criticism for essentially just being a web wrapper and having some things missing. The biggest disappointment for many was that you could no longer sign in with a brand account or switch channels as you could in the previous release. Google said they would be brought back soon and has just made good on that promise with a new update.
After several users complained about the lack of account switching on YouTube's help forum, it was escalated and appears to have been solved with version 2.01.04 which is beginning to roll out now. It doesn't look like there are any other meaningful additions in the update, and the changelog purely states that "Brand accounts are back."
A number of users will be thrilled with this news – some even had to uninstall all updates and use a much older version of the app in order to get around the problems. The new update is starting to roll out to people, but it will take a while to reach everyone, so if you really can't wait you can install it manually via APKMirror.