Google Uses Multiple Signals to Choose HTTP or HTTPS URLs
Redirects, internal links, sitemaps, rel canonicals are taken into consideration when Google chooses to index a page on HTTP or HTTPS. Internal links to HTTP URLs after a migtration to HTTPS will give Google conflicting signals, but usually there will be enough signals to indicate that the HTTPS version should be indexed.
Mixed Migrations May Cause Google to Index HTTP or HTTPS URLs
Forgetting to update your sitemap files following a HTTPS migration could cause some pages to be indexed with the HTTP URL and some HTTPS.
Only Change Protocol During an HTTPS Migration
An HTTPS migration can be processed much faster providing only the HTTPS protocol changes, and the domain, URL path and parameters don’t change.
Most HTTPS Migrations Take a Day to Change in Index
A HTTPS migration is easier for Google to process than most other types of migrations because it keeps the same domain and same URLs. If a site is restructured with changes to internal linking or the domain name, it means Google has to think about a lot more. However, HTTPS is still a big change and takes time to be processed by Google – most take a day or so to switch over in Google’s index.
Move All Pages at Once For Migrations
John recommends moving all pages over at once for migrations. If the migration is done in steps, it tends to take a lot longer and there’s more room for error with Google struggling to understand if you’re migrating part of the site or the whole site.
If Site Doesn’t Have Valid SSL Certificate Google Will Stick to HTTP Version
If Google discovers an HTTPS URL but it doesn’t have a valid certificate then they’ll probably stick to the HTTP version.
Google Sees SSL Certificates as the Same as Long as Have Valid Certificate
Google sees all SSL certificates as the same as long as they are valid and accepted by modern browsers.
Server Performance and Robots.txt Can Impact HTTPS Migrations
An HTTPS migration might have problems if Google is unable to crawl the site due to server performance issues or files blocked in robots.txt.
HTTP to HTTPS migrations should be recognised within a couple of days
If you do a clean one-to-one redirection to HTTPS, you should see minimal fluctuation of rankings and fairly stable traffic. If you’re seeing an impact on overall rankings and traffic, you should notify Google.