Quick update: The administrators/moderators/users are very happy about having moved from phpBB to Discourse. A lot of people came back. New registrations too. So, indeed a success.
Fun fact: they use the anniversary plugin to manually mention the birthday of every member every day on a dedicated topic.
I thought at first it was a bad idea, but it made people come back and it seems to be working well to this intent. I wouldn’t advise it for every community though… 
I’m a bit surprised they haven’t activated the chat yet since all is ready and configured, but I know they intend to do so. I’d like them to activate it and see how it goes in this community, so I can make feedback about it here.