Although we do not recommend it, changes can be made as long as all three conditions are met simultaneously:

  1. More time
  2. More budget
  3. Lack of scheduling commitments with other clients.

But trust me, we won’t need to get into this situation.
If we both do our homework, you will approve the still images before creating the video.
This way, we will avoid last minute changes and cost overruns that only lead to unpleasant situations right at the end of the job.