1.) Locate the current intro video called "aoeiide_titlevideo.wmv" (AoE2DE\resources\_common\movies)
2.) Replace it with the old intro
=> The easiest way is to replace it with the HD Edition intro (Age2HD\resources\_common\movies) -> don't get confused as there are two video files, both are the same except "aoeiihd_x2.wmv" is for the The Forgotten expansion
=> However, you can also download it from YouTube using a Video downloader.

Remember to rename the files!