There are several reports of migration to HTTPS (using 301 redirects and HSTS) without an impact to rankings (no fluctations).
Since that seems like changing URL structure I'm thinking to migrate
to more friendly URLs like
But Google do not really recommend changing the URL structure:
https://www.youtube.com/watch?v=YL4sdTcUJzo#t=1301 (they say fluctuations are possible)
Are there some recent tests (user cases) which show that it might not be a good idea to migrate the URL structure and only do HTTP to HTTPS URL change?
SeoMoz to Moz migration showed that 301 change of domains and URLs went very smoothly.