You can replace http with https and the forum will work. For a variety of reasons, making the forum entirely use https is complicated (if you switch yourself, your browser may warn you that some things on the page are secure and some not). Because we have advertisers who serve images from their own servers insecurely (http vs https), we will not quite make it to full security in our present state. It is on the radar, as search engines will eventually punish sites which do not switch.