I had only my main page changed to HTTP to test out if I want to change from HTTPS to HTTP. All my backlinks are to my HTTPS site. Will I lose all of my backlinks and have to start over when switching to HTTP?
Google will switch between https and http (and vice versa) without any noticeable link juice loss - as long as the links (apart from prefix) are identical.
However if you want to make sure that your links return to http, and not leave it to Google or chance, then a 301 redirect will force the change.
Note that Google has stated that their Googlebot now automatically tests to see whether it is possible to serve pages via https without issue, whether you include hard coded https links or not, however they won't override a hardcoded http links.
They have also stated that they are beginning to provide a small preference to sites served via https so if your site is correctly setup for it, then it is preferable for rankings.
Basically if your site is setup correctly for https I am not sure why you'd want to go back to http as a secure connection increases trust for your customers and is preferred by Google.
LetsEncrypt and services such as Cloudflare provide free trusted certificates for your site.