Just to provide a counter-point, for any future readers of this thread:
When we moved our community to Discourse in November of 2016, we had ~110,000 registered users and close to 2 million posts. The old forum was at that point exactly 20 years old, and had been running on the same home-grown software the whole time (with incremental fixes as we went along, of course).
Our community is one of Jaguar car enthusiasts. The type that will argue for months over the correct color of the sticker inside the factory tool kit bag for a 1961 E-Type OTS. You know, people who care about the important things in life.
Needless to say, the differences between our 20 year old cottage industry forum software and Discourse were dramatic. Of course we had complaints, and yes, contributions slowed for a while (we’re based on voluntary contributions, not ads). But it wasn’t as bad as it could have been.
One of the things we did was to set up a “demo” Discourse site in parallel with the old site, and import about a month’s worth of traffic into it, so it didn’t look too much like an empty mockup. Then we invited our users to log on to the demo site, play around with it, and give us feedback. I think that step alone saved us a lot of misery.
We also had a volunteer (now one of our moderators) play around with Discourse before we went live, and then be on hand to answer questions from those who just couldn’t figure out how to use the new software. Lo and behold, after a few months, the complaints had ceased and the contributions were back up to their old level.
Of course, your community could be very different from mine, I have no idea. I’m just saying that it is possible to get a community of mostly grumpy old men, all of whom are averse to change, and most of whom wouldn’t know the difference between a modem and a modal dialog box, to get comfortable with Discourse. In fact, I think now they enjoy it. Not only that, but we’re growing again as a community, after having stagnated for years.
So, it is possible to have a successful transition to Discourse, but it may require some planning. Also, getting feedback from the community before making the plunge can be crucial.