And what happens to users sitting on the site when the admin DISABLES HTTPS?

Question

Apache HTTP Server

We have a website.
There is a page on it. It has a form.
And here on this page the user sits. Fills out a form. The form is going to be submitted to a URL with https://. Here the user clicks submit... And right before that, the admin disables HTTPS!

What's happening?

If the admin, on the contrary, included https:// - then the answer is clear - the http:// POST request would return 302, redirect to https:// and everything is OK, and then it depends on the browser (probably, he will remember this in address bar will suggest https://)

But what if it's https:// > http://?