Thanks Jan – before I go through the process of changing all file and database references, I believe I have noticed the source of my problem:
When I look at a media image, the ‘perma link’ is showing as https. However, the ‘file url’ is showing at http. I cannot manually change the ‘file url’ http to https on the wordpress media image edit page.
Is there a fix in word press where I can upload an image as HTTPS, instead of HTTP?