It depends what forum you were using before... we have specific exporters for most of the major forum software platforms.
If it's custom then you're looking at having to write your own or contact us at email@example.com for a quote.
We are currently on phpBB, works great but it's old. No mentions, mediocre mobile support, no notifications, no updates between pageloads, etc, etc.
I think future users would really like NodeBB but our current users are extremely accustomed to the current look and feel of the site.
Because of that I'm really very curious about your experiences with moving from an 'old' forum platform to NodeBB. How did you prepare your users? How did they react to the plans and to the actual move?
What happened to your pageviews/sessions/bouncerate/session time/etc after the change? And what happened to your organic traffic? Did you setup proper 301s from the old locations?
Any hints and tips for a prospective convert?
A full migration has a number of moving parts, and if you have a vested interest in maintaining SEO then 301 redirects from the old forum are a must.
That said --
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