Enabling SSL will result in many threads being marked on the browsers as insecure. To get the proper green SSL icon in your address bar, everything on the page has to be SSL. If there are links to non-ssl images, the page does not get the green SSL icon, and if you left click on the greyed icon, it will say "This connection is not protected".
If you look into the details, it will say something like "This site has unprotected content".
If there is a link to an image on a remote site, the address bar SSL icon will be green as long as the remote image address is also SSL.
Probably impossible to get around this with old threads, and for new threads, to get the green SSL icon, you would have to insist on https:// links only.
Edit: many users have uploaded images, and then posted the image address into their post to get the full sized image. All of these addresses start with "
http://www.eevblog.com/....", so they will force to SSL icon to grey. It may be possible to write a script to find these links, along with links to other posts, and turn them into relative addresses to make them SSL compatible.