Can I use javascript to dynamically change a video’s source?

Sure,

1. You can set the src attribute on the source element:

   document.querySelector("#myVideoTag > source").src = "http://example.com/new_url.mp4"
   

   Or using jQuery instead of standard DOM methods:

   $("#myVideoTag > source").attr("src", "http://example.com/new_url.mp4"​​​​)​
   

2. Then you need to call the load method on the video element:

   videoElement.load()