SEO Aspects of Moving from http to https
Once upon a time the question of whether to switch from http to https would rarely come up. Most sites didn't need to include all pages under their domain into https and only pages that included receiving sensitive information were under that protocol. But these days are long gone and today just about every big client I have is considering this move. The reason for this is the switch Google has made to https about a year ago. But we'll get to that soon enough.
Let's start by first giving a general overview about https, for those of you who come from a less technical world. Https is a web protocol that offers a stricter security level than what the classic data transferring protocol - http (hypertext transfer protocol). The extra S in the end simply stands for 'Secure'.
Usually this protocol is used solely for transferring sensitive information through the web. However, Google has managed to add another use to this protocol. They've switched their domain to https for every logged in user. The official purpose of this switch was allowing the users more privacy. What this move actually means is - when a user reaches you from Google's organic results, while being logged into any Google account, they will reach you from https protocol. If your site defined with https as well, data about this user, including the keywords he used, are lost both in Google analytics and in your site's traffic tracking logs. To solve this problem, Google has offered switching your site to https. You still wouldn't get this information in GA, but you at least would have it in your site's logs.
The answer seems simple then - switch to https and all is well. The thing is, when you make such a change there are consequences you need to take. When the owners this site came to me with a request for this move, I started reading a little about it. I was trying to learn how this switch would affect the extensive SEO work we've already done on this site. What I've discovered was more than alarming.
The main issue was that http domains are not considered the same as https domains by Google. This means that switching to https is almost like changing a domain name or maybe closer to suddenly adding www to your domain. This would require some steps to be taken like redirection or canonical links. Also, all links to site would need to be changed to the new url structure, including inner and incoming links. Once the switch has been made, you're likely to have a drop in your ranks for most of your ranked terms. This could be either temporary or permanent. Google can't guaranty which it would be for your domain.
Another big influence on the site would be its performance - since we've loaded the servers with a lot of redirects and since the redirect itself takes time, you can expect a definite rise in loading time for all site's pages.
But even if we put the SEO issues aside, there are other problems to consider. For instance, you've managed to solve the data collecting from https sources, but you're going to have problems with data collecting from http sources.
In short, with the number of negative outcomes you get when switching to https, as a site owner you need to think very carefully if it's really worth it. As you can see in the example I've included, the site's owners were ultimately convinced not to take this step and they kept their http version as is.