It requires re logging in and connecting your ST using the new method. Unfortunately it doesn’t seem to work. I tried it several times, along with others. There is a thread regarding this already: https://community.smartthings.com/t/update-for-the-harmony-app-to-work-with-the-new-smart-things/260071/6
Also, on the Harmony subreddit, they are discussing a firmware update that has bricked many remotes and causing a slew of problems.