This is a quick and easy thing first thing you need is the old video as a bik file which I have provided here:
https://drive.google.com/file/d/1YIsqHerofVzMOOqp564OaD8h58hMwSBe/view?usp=sharing

Steps

• Download old BIK
  
• Go to KF2 files
  
• killingfloor2/KFGame/Movies
  
• Backup the current MenuBG.BIK in case you want to go back
  
• Add the file you downloaded there

And now when you play it should be old background.

Comment if you need any help.