What is the actual process by which academic ethics change?
I'm sure both processes happen.  

The latest Common Rule for human subjects is a great top-down example.  https://nexus.od.nih.gov/all/2019/01/07/nih-implementation-of-the-final-rule-on-the-federal-policy-for-the-protection-of-human-subjects-common-rule/ 

This process reinvestigated human  subject ethics in the context of modern-day genetics.  It almost required specific informed consent for the collection of deidentified tissue, with periodic renewals of consent and the ability to yank consent.  Picture what that would do to say, developed perpetual cell lines used for research!  MANY universities were waiting anxiously to see what would come out of that process and there were very large discussions on how such requirements would be met.  We even modified our CLINICAL operations to accommodate anticipated changes that never happened!!

I say this is a great top-down example because the ethics are, and still remain, hard to figure out.  Stakeholders had to get together and form consensus.  Nobody knew what the outcome would be when the process started.

As an example of bottom up, there is the example of neurosurgeons that now require sham surgeries before adopting a new procedure. This fell out of human fetal tissue transplant attempts. ncbi.nlm.nih.gov/pmc/articles/PMC1733639  There is no relevant reg, but I'm sure this is now incorporated into RSRB discussion.