You don't need to override migration, you just need to add a field in product review table.
So, first of all, create a package, create a migration file in this package and add the desired column in it.
Now you need to override view file from where we will submit product review so that you can provide a field to attach video.
You can follow given a link to override views - https://forums.bagisto.com/topic/221/how-to-override-view-file-in-bagisto/2ow
Now you need to create the same name route in your package as the shop (PACKAGE) has for review post calling with your controller ( WHICH YOU WILL CREATE IN YOUR CONTROLLER).
now all data will become in your controller, you need to write your own login here for the video upload.