It's not impossible to migrate Redmine to Flarum or any other new forum software. However no tools appear to exist so we would need to author something from scratch. In a more established project team we'd likely have a resident python genius itching to take on the task, but our 'team' is small and still in the early stages of finding its feet and there isn't one. The alternatives are finding and paying someone to create the tools, waiting an unknown time until we manage to recruit someone suitable to the team, or simply grandfathering the data; which requires the least effort, no money, and moves the project forwards quickly. IMHO it's not a hard choice.
Redmine is made for the corporate world and defaults to first/last name identities so there's a few people whose identity will change with the switch to username (Jonas will be one of them) but the main impact is aligning their Tvheadend identity with the one used publicly in other forums. In end-user forums nicknames are normal and look friendlier, so I expect a few complaints but see it as an overall positive move.
Forums do have hierarchies and little status symbols. Everyone will start with a zero post count, but those who post will quickly reassert themselves and we will migrate the registration dates so 'old hands' can still be seen as such. I've skipped creation of user tiers as Redmine doesn't have them but Flarum allows proper 'Moderators' (with real moderation tools) with some visual cues on roles, and anyone who donates through OpenCollective should be auto-tagged as a sponsor if they use the same email address on both sides. If we want to add forum things in the future we can.
Kodi attempted bulk password reset notifications after their forum breach last year and it's considerably harder than you think to one-time send a large volume of identical emails to a global userbase and have them reliably delivered to inboxes (most end up in spam). It will also cost us money with Mailgun (or any alternative service; we'll be over the limits). Some sticky posts in the forum news section will suffice.
If you start poking around in the Redmine user DB some of the spam-registration patterns are quite clear so we'll skip migration for ~55% of all current users. I don't see value in doing a further cull in 6-12 months time. In my experience users often take a multi-year hiatus from forum visits; the "if it works, don't fix it" principle is strong and users only visit when something breaks. If we do a good job people don't visit often. If we want to forcibly engage our audience, we can have an auto-update feature and push something buggy :)
I'm tenatively planing downtime for Monday. Partly because more people are likely to use the forum over the weekend. Mostly because I'm running a half-marathon tomorrow and will be tired after (never a good time to do something important).
