The engine goes "live" on test in March, but if you read both of the dev posts in the linked thread, it's going to be a slow port from the old system to the new throughout the year. Their goal is to have a port of the default UI completed by the end of the year. There are conversations happening about a new default UI, but we won't be seeing anything on that until 2024 (or possibly next beta) most likely.
There will be time to work out the kinks.
Will there be changes to what we can do as custom authors? Yes, most likely. Will there be kinks? Yes of course. I'm still more excited than intimidated personally.
You're welcome mh272, I figured there'd be other authors like myself needing to brush up on their html skills.