# Do I need to set something in Search Console? No, just add the HTTPS site there. The change-of-address setting doesn't apply for HTTP -> HTTPS moves.
# How can we do an A/B test? Don't cloak to Googlebot specifically, use 302 redirects + rel=canonical to HTTP if you want to test HTTPS but not have it indexed. Don't block via robots.txt . More about A/B testing at https://googlewebmastercentral.blogspot.ch/2012/08/website-testing-google-search.html (302 redirects aren't cached.)
# Will the rel=canonical guarantee that the HTTP URL is indexed? No, but it's a very strong signal when picking the indexed URL.
# What's the next step after testing? Follow our site-move documentation ( https://support.google.com/webmasters/answer/6033049 ). Use 301 redirects from HTTP to HTTPS, confirm the new version by adding a rel=canonical on the HTTPS page, pointing to itself, and submit sitemaps including both HTTP & HTTPS URLs with new change-dates (in the long run, just keep the HTTPS sitemap).
# What about the robots.txt file? The HTTPS site uses the HTTPS robots.txt file. Check that it's reachable or serves a 404 result code, and check that your HTTP URLs aren't blocked by the HTTP robots.txt file.
# Is it OK to have just some pages on HTTPS? Yes, no problem! Start with a part, test it, add more.
# Should I move everything together, or is it fine to do sections? Moving in sections is fine.
# Will I see a drop in search? Fluctuations can happen with any bigger site change. We can't make any guarantees, but our systems are usually good with HTTP -> HTTPS moves.
# Which certificate do I need? For Google Search, any modern certificate that's accepted by modern browsers is acceptable.
# Do I lose "link juice" from the redirects? No, for 301 or 302 redirects from HTTP to HTTPS no PageRank is lost.
# Will we see search keywords in Google Analytics when we're on HTTPS? This won't change with HTTPS, you can see the search queries in Search Console.
# How can I test how many pages were indexed? Verify HTTP / HTTPS separately in Search Console, use Index Status for a broad look, or the sitemaps indexed counts for sitemap URLs.
# How long will a move from HTTP to HTTPS take? There are no fixed crawl frequencies, it depends on the size of your site, and the speed of crawling that's possible. The move takes place on a per-URL basis.
Hope this helps clarify some of the open questions! Let me know if there's anything missing.