Turn off https in Wordpress 4

I have a site that uses wpmegamenu to direct people to several non-wordpress pages in an iframe.

I just upgraded to 4.0.1 and it moved everything to https. (which is great!) But this means that the pages that get shown in an iframe are blocked as a mixed content error.

Is there a way to turn off https and have all the fun of 4.0.1 without the https? (So that we can upgrade the pages that lag behind in security without holding up the man thrust of the action.)

I've tried changing wp-config.php and adding define('FORCE_SSL', false); but it made no difference.

There is an option in the settings page wordpress-https_secure_external_urls but it is serializd. It seems to be a place to whitelist http pages, but I have no idea how to get urls onto that list.