I guess some backlash should be expected, new users won't suddenly come out of the woods, so you can only lose some of your active users in the first period after the conversion.
But can we expect 5%? 20% 50% decrease in traffic? And what's to be expected of the situation a few weeks or even months after the conversion? We would move because we think users will stay engaged more with notifications, mentions, modern looks, better usability. We also expect new users to sign up in bigger numbers because this will be easier (SSO) and also because of the improved usability and better looks. But that's all theory so far.
Some real life stories about conversions of big boards would be most welcome 🙂