How to achieve this type of transition? I've been looking everywhere to try to figure this out. I am trying to figure out how to make a video transition where the background changes before everything else. Is this just done with a mask? 

https://instagram.com/p/BF-K-V5ges1/
According to the comments on that page, it is rotoscoped - which is (from Wikipedia)


  the technique of manually creating a matte for an element on a live-action plate so it may be composited over another background.