It is possible to go back change the status of your plan review after you have completed it. However, this can only be done while the current review round it's still underway by pressing the 'More' button and selecting 'Change Result'. If all the reviewers have already completed their respective reviews, then it won't be possible to make changes to the review.
As an example: the below review cannot be changed because both reviewers have already completed their reviews, meaning the current round is completed. To change the review a new round has to be initiated.
On a different example, either review can be changed because the review round is still underway as one of the reviews is yet to be completed:
Also worth noting that, once a Plan Review workflow step is completed, it cannot be resumed or changed. If you need additional review after completion, you'll need to add another Plan Review workflow step to perform the necessary review work.
💡 Would you like to learn how to edit the workflow of an application? Check this article out!