you simply redirect to the https version. your ranking will actually improve.

-h

your ranking won't improve that much. adding security to your site will boost your site up. google likes trustworthy sites. It adds bonus points to your trust quality metric in google's algorithm.

do a 301 canonical on your http domains immediately after you add the security.

Moving your website from HTTP to HTTPS is very similar to migrate your web site to a new URL structure, or even moving to a new domain. Past experience has told us that there are so many things that can go wrong if things are not implemented correctly

Just 301, it won't affect your rankings much.

Of course it will affect your ranking - on a positive manner.

Basically, HTTPS is a secure domain, unlike HTTP. Google likes secure websites which are well-taken care of. It's actually recommended to switch all of your domains to the secure one, though I'd call up an expert on the matter to do the job for me if I were you. Why? Well, you'll probably encounter a host of issues if it's not implemented correctly.

Domain with https is secure websites and Google loves secure website. You simple 301 redirect your website and your ranking will be increase.

I think there is no issue in changing the Url. There is no change on the ranking of the site. so you change your url.

Last year Google announced that https was going to be included as 1 of the over 200 ranking factors they use. However, every test, case study, or experiment I have seen someone try in testing this has shown no rank improvement.

It might be a ranking factor, but everything points to it being a negligible one.