Given that Greg trained as an MD because he wanted to do good, this here probably counts: https://80000hours.org/2012/08/how-many-lives-does-a-doctor-save/

(and the many medical doctors and students who read posts like this and then also changed their minds, including me :-) )

Shutting down No Lean Season:

https://www.evidenceaction.org/were-shutting-down-no-lean-season-our-seasonal-migration-program-heres-why/

https://blog.givewell.org/2019/06/06/evidence-action-is-shutting-down-no-lean-season/ 

A personal example: I wrote Should Global Poverty Donors Give Now or Later? and then later realized my approach was totally wrong.

Why Anima International suspended the campaign to end live fish sales in Poland

https://www.jefftk.com/p/revisiting-why-global-poverty

Ajeya posted an update to her AI timelines report:

My personal timelines have also gotten considerably shorter over this period. I now expect something roughly like this:

• ~15% probability by 2030 (a decrease of ~6 years from 2036).
• ~35% probability by 2036 (a ~3x likelihood ratio^[3] vs 15%).
  • This implies that each year in the 6 year period from 2030 to 2036 has an average of over 3% probability of TAI occurring in that particular year (smaller earlier and larger later).
• A median of ~2040 (a decrease of ~10 years from 2050).
  • This implies that each year in the 4 year period from 2036 to 2040 has an average of almost 4% probability of TAI.
• ~60% probability by 2050 (a ~1.5x likelihood ratio vs 50%).

This doesn't have all of the properties you're most looking for, but one example is this video by YouTuber Justin Helps which is about correcting an error in an earlier video and explaining why he might have made that error. (I don't quite remember what the original error was about – I think something to do with Hamilton's rule in population genetics.)

Students for High-Impact Charity shut down based on their results:

• Within a year of instructor-led workshops, we presented 106 workshops, reaching 2,580 participants at 40 (mostly high school) institutions. We experienced strong student engagement and encouraging feedback from both teachers and students. However, we struggled in getting students to opt into advanced programming, which was our behavioral proxy for further engagement.

Joe Carlsmith's report on power seeking AI says this (emphasis added):

I assign rough subjective credences to the premises in this argument, and I end up with an overall estimate of ~5% that an existential catastrophe of this kind will occur by 2070. (May 2022 update: since making this report public in April 2021, my estimate here has gone up, and is now at >10%.)

I changed my mind about promoting veganism.
https://www.vox.com/videos/2017/5/27/15701168/save-animal-lives-eat-beef-not-chicken