Sans serif -> serif change is okay with me, but they are a bit less emphatic than before (maybe with less space?), so especially level 3+ headings now don’t seem quite as much like new sections. Personally I think I’d prefer slightly more space above the new headings. Currently seems like a bit less than 0.5 lines extra (?) which feels a bit tight to me (see picture). YMMV. [Note, I appreciate the lack of extra space below the headings. That’s a decided improvement vs. the previous style. I wouldn’t mind if level 2 headings also had no extra space below.]
Out of curiosity, is the thought process that goes into these kinds of global style changes written down anywhere?