Probably not super helpful/what you're looking for, but one broad category of groups who go from 'doing violence' to 'doing much less or no violence' (often within a short space of time) are resistance organisations that successfully manage the transition, usually after achieving some level of progress. The ANC in South Africa seems like a good example. Sinn Féin in Ireland (established as the political wing of the IRA) is another.
I’m not quite sure what you mean by “deeply painful process.” There is often a segment of any community that resists any change. That’s not to say that it has to be a fight, but community practices have an inertia to them. Sometimes that a shift that’s happens over time.
For instance, when I was a kid (1980s), “gay” was a common pejorative. While there have been plenty of painful events that have happened in the lives of LGBT folk, I don’t think that this was due to some process that is deeply painful, other than people slowly changing their minds over time.
I’ve seen the polyamorous community shift best practices over time. Again though, I don’t think that this is due to some inherently painful process. One could argue that the collective pain that we experience as we’re making mistakes is that process, but I suspect that isn’t what you mean here.
I think that change is generally hard, but it naturally happens over time.
One issue is going to be filtering.
Strife and conflict is memorable. So you are searching for the least noteworthy examples, the ones that people are least likely to comment on or remember.
I don't know what qualifies as a "community" really. At work I have seen uncontroversial changes come in a few times.
To be more explicit about my model, I see communities as a bit like people. And sometimes people do the hard work of changing (especially as they have incentives to) but sometimes they ignore it or blame someone else.
Similarly often communties scapegoat something or someone, or give vague general advice.
I've been thinking about community improvement and I realise I don't know of any examples where a community had a flaw and fixed it without some deeply painful process.
Often there are discussions of flaws within EA with some implied notion that communities in general are good at changing. Maybe this is true. If so, there should be well known examples.
I would like examples of communities that had some behavior and then changed it, without loads of people leaving or some civil war.
Examples might include:
Also if anyone knows of literature they trust on the subject I'd be interested in it.