Changing a site from HTTP to HTTPS is considered (by the search engines) to be a site move. Changing the structure of a site at the same time as implementing https is a sure way of losing organic traffic. To implement https, first ensure your HTTP and HTTPS sites are identical.
Implementing HTTPS can be a great business move, but if you try to do too much the search engines will react and you are almost certain to take a major hit.
A site migration will almost always result in a temporary loss of traffic — search engines need time to process the change and update accordingly. A well-implemented migration can minimise traffic fluctuations and in time Google will treat the new site as if it were the original.
Migrating to https offers user security (when filling in forms etc) and there should be a slight SEO gain, but there will be a period of transition.
Never do a site migration without first testing everything on a test server. Verify that the redirects work properly and do all of the checks before going live.
A well-planned and monitored migration shouldn’t permanently affect your traffic, but you should plan for a temporary dip. A move is best performed during a slow time of the year (think bars/restaurants in January!)
Benchmark exactly what you have before the move
Make a copy of your Google Analytics data. You will need this information so that you can quickly identify if any traffic is lost after the migration. Also, information from Search Console for your http property should be downloaded for analysis after the move.
If any traffic is lost, export the Analytics data from your new site and run a side-by-side comparison with the data from your old site, so that you can identify precisely which pages lost the traffic. In many cases, a loss of traffic will be isolated to individual pages, rather than taking place across the entire site.
As mentioned, resist the urge to make any other changes when implementing https. The URL architecture should be identical to the old one. A site migration may seem like the ideal time to make structural changes, but you should be aware that doing so may cause Google to see it as an entirely different site. Also, if you do both at the same time, you will not be able to determine whether any losses in traffic is a result of changing the architecture or https.
Update all internal links
The HTML links on your new site should point to the new https site, not the old one.
This might sound obvious, but as you go through the process, you will quickly realize how tempting it might be to leave the links unchanged, since they will redirect to the new URL anyway. Do not succumb to this temptation. Apart from the server load, which slows down site performance, the redirects may dampen your PageRank.
The ideal way to rewrite the links is by performing a search and replace operation on your database – wordpress has a plug in to make this easy.
Canonicalisation tells search engines which web page is the original and should be indexed in the case of having duplicate pages. If you have identical http and https pages (that you will have), make sure the canonical is set to the new version (https) on every page.
In combination with the redirects, this tells Google that the new site is, in fact, the new location of the old site. URL parameters create duplicate content that should always canonicalize to the parameter-free URL.
If both multiple versions of a URL are published, it results in duplicate content. Canonicalization should take care of the issue, but also set up redirect rules in .htaccess so that only one version of the page is accessible.
Make sure that links are consistent to avoid redirects from internal links.
Verify that only HTTPS or HTTP is used and that only the www or non-www version of the site is accessible. The others should redirect to the proper site.
If your site has a search function, the search result pages should be noindexed.
Update and submit sitemaps
https will need a new property setting up in search console. Do this, update your sitemaps for https and submit for indexing.
Use a regex expression in the .htaccess file of your old site to redirect HTTP to https. The regex expression should simply swap out HTTP for HTTPS.
Test your redirects on a test server and verify that this works as expected.
Keep in mind that once the redirects go live, your site has effectively been migrated. The new site should be in pristine condition before setting up the redirects.
Monitor traffic, performance and rankings
Keep a close eye on your search and referral traffic, checking it daily for at least a week after the migration. If there are any shifts in traffic, dive down to the page level and compare traffic on the old site to traffic on the new site to identify which pages have lost traffic. Those pages, in particular, should be inspected for redirect errors. You may want to pursue getting any external links pointing at the old version of the page changed to the new one, if possible.
It is equally important to keep a close eye on your most linked pages, both by authority and by external link count. These pages play the biggest role in your site’s overall ability to rank, so changes in performance here are indicative of your site’s overall performance.
Mark dates in Google Analytics
Use Google Analytics annotations to mark critical dates during the migration. This will help you to identify the cause of any issues you may come across during the process.
Update PPC and all other platforms
Update all of your PPC accounts, social media profiles, bios, other websites you own, forum signatures, and any other platforms you take advantage of, so that the links point to the new site and not the old.
Monitor your indexed page count
Google will not index all of the pages on your new site immediately, but if the indexed page count is not up to the same value as the old site after a month has passed, something has definitely gone wrong.
Keep all of the above in mind if you are planning to migrate your site to https, and it should go off without a (permanent) hitch.