Anyway, so I decided to update these links with a MariaDB regexp_replace rather than manually updating every one of these smilies. In retrospect, I'm not sure whether that may have caused the issue I'm now seeing: Threads are still using http for the img src of the smilies! I've tried rebuilding some of the caches, but no luck so far. I didn't really see a cache that seemed to apply to smilies? I thought maybe it was the Thread Information one, but no joy.
When I run Rebuild Attachments Thumbnails, I get:
ErrorException: Fatal Error: Maximum execution time of 120 seconds exceeded - library/XenForo/Image/ImageMagick/Pecl.php:310