Ok, I’m not trying to sound harsh but your project is a HD DVD project, somewhere along the way you clicked on that settings. There is NO way back to SD, you have to rebuild the project.
You need to be a little more precise about your descriptions, they don’t make sense. Nothing is “SET” to Video Standard, Video Standard is a line description and the setting is either SD or HD DVD, after it is set to HD DVD it becomes grayed out because you cannot change it back to SD.
Once again your project may have started out as SD but it is now HD, which can have both HD and SD material in it, but it will not play on a standard DVD player anymore. How did it change, you got 3 options, 1) you unknowingly changed it at some point. 2) someone else came in and changed it on you. 3) the program did it all by itself. #3 is highly unlikely.
No shame in making a mistake, we all do it, in fact I think I made one a couple of years back. 🙂