yes a redirect 303 seeother should work, as bookmarks should be forwarded. You will be best to notify visitors to update their bookmark, as when the ssl key runs out ...

alternatively if the ssl key cost is the issue, then with a little work startssl.com can give you class 2 ssl keys for free, and you customer confidentiality can remain ssl secured.

if you have used HSTS (Strict-Transport-Security) then after switching to non https could give your returning visitors issue as they would have to delete their browser cache else they will see warning on their browser.

First of all, what web server are you using? If it's nginx, you can add redirect like this:

server {
        listen 80 443 ssl;
        server_name example.com;

        if ($scheme = https) {
            return 301 http://$server_name$request_uri;
        }
        ...
}

In case of Apache it would like this:

<VirtualHost _default_:443>
   ServerName example.com
   DocumentRoot /usr/local/apache2/htdocs
   SSLEngine On
   ...
   Redirect permanent / http://example.com/
</VirtualHost>

But you would probably need to completely rework your config since you switch from Drupal to WordPress. But if you don't want to bother with configuration, you might want to take a look at Wodby on the Projects page.