Once your site is on HTTPS, you never want to drop HTTPS support. Doing so would be bad for security, frustrating to users, and catastrophic for SEO.
Once you drop HTTPS support, there is no way to redirect your HTTPS URLs to their HTTP counterparts. Just to be able to redirect, you need to run an HTTPS server with a valid security certificate.
If you turn off HTTPS support, your HTTPS URLs will start misbehaving.
- If you are not running an HTTPS server, connections to those URLs will be refused and error out.
- If you are still running an HTTPS server, but don't have a valid certificate, those URLs will present "Invalid Certificate" errors. The following cases would be similar:
- You let the old certificate expire and leave it in place. Users would get an "Expired Certificate" error.
- You remove the certificate and the server falls back to a default self-signed certificate. Users would get an "Un-trusted Certificate" error.
- You remove the certificate and the server tries to use a certificate from some other site hosted on the same server. Users would get a "Certificate Name Mismatch" error.
- If you continue to run a HTTPS server with a valid certificate to be able to redirect to HTTP, you might as well just continue to support HTTPS fully.
Because HTTPS URLs will show errors when you drop HTTPS support, it will hurt usability and SEO:
- Users that get to your HTTPS URLs through bookmarks, links, or browser history will get an error page and have no easy way to get to your HTTP site.
- Search engine bots will encounter errors at all your HTTPS URLs causing them to drop from the search results. Because there will be no redirects, search engines won't be able to index your HTTP URLs in their place. Your HTTP site will eventually get indexed, but with much worse rankings.
HTTPS is not a strong ranking factor for Google. Google gives a slight boost to HTTPS sites over HTTP sites. Sites that have moved from HTTP to HTTPS have generally found no meaningful ranking boost from doing so. Most of the SEO problems from dropping HTTPS support will be due to the inability to implement redirects rather than from search engines using HTTPS as a ranking factor.
There is no good reason to drop HTTPS support these days. Many web hosting companies offer it for free by obtaining free certificates from Lets Encrypt for your site. You can even add HTTPS to a site by signing up for a content delivery network (CDN) and having the CDN implement HTTPS for you.
If you already dropped HTTPS support and want to re-enable it, your rankings will likely recover to some extent. If HTTPS has only been off for a few days, I would expect rankings to fully recover in a few weeks. If it has been months, it could take a year for rankings to recover. In other words, I would expect it to be similar to other types of outages where Google completely forgives outages up to 24 hours but rankings take a few times as long as the outage to recover for longer outages.