This guide details the steps to help you to change your plan on Cloudflare using the Cloudflare API. You can always change your plan within the user interface of the Cloudflare portal, however, you may be looking to change your plans in bulk and that is where using the Cloudflare API makes more sense.
For those trying to proxy your traffic through Cloudflare and are also using Blogger.com/ Blogspot.com to host your website you may find some issues with the HTTPS redirects and general loading of your HTTPS on your website.
This follows some of the posts that I have seen on the Cloudflare Community page on "Custom domain with blogger not working".
I recommend setting up the redirect on Cloudflare.com, so that Cloudflare's edge locations will do the redirecting effort, rather than you needing the visitor to send a request back to your origin as part of the redirect work.
So here's a few things that I did to have this work.