I agree revising the plan is not working backwards, it is just a response to new knowledge. However, it is important to discover that knowledge beforehand if possible. That is why extensive measures should be taken during the planning phase so that no revisions must be made unless absolutely necessary. In this case, it appears this could have been easily determined before the initiation phase which would have prevented the need to go back and revise. Nevertheless, it is better to discover issues sooner than later, address them, and pivot accordingly, rather than ignore them altogether.