Well, just back-up your site and play with the code. If you screw it up, just replace the file and try again. In my experience that’s the best way to learn.
Compare the source code of you published pages to that of your posts. Also, look through the files to see if they maybe named the post template something different. Probably not though.
If the page template is used for the posts as well, then you’ll have to edit that.
I had the same problem as you did. You have to find with which ID your theme calls the featured image. It’s different for every theme. In my case it was ‘post-thumb’
Once you have that, look to the previous page of this thread for a reply by Mike Bikkel. He explained to me how you can turn the original featured image function into a conditional funtion which calls the featured video if it is there. Replace ‘post-thumb’ with whatever your theme’s featured image ID is.