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.