I have just this week updated my own system from 4.2 to 4.3 (since I wanted to try the new Dashboard https://www.univention.com/products/univention-app-center/app-catalog/ucs-dashboard/) and for me it worked with the documented steps.
What do you mean with “errors in GUI”?
No reinstall is necessary, as this only concerns the Kopano packages (and these will not be touched by the app update, since they are already newer). Like I said before the app will just upgrade the integration packages.