This is not possible, because in Scrum, after a Team has commited to a set of Stories, the Product Owner should no longer be able to interfere with the Team's self-organization and work.

By changing the priority, the PO could sabotage the Team's self organization: Today, the Team would start working on Story A (that has a high priority), but right before they complete it, the PO changes the priority. Not Story B is of higher priority than Story A and the Team needs to shift their work to Story B. Unfortunately, they have taken up too much work during Sprint planning and cannot complete Story B in time.

Now, both Story A and Story B are 90% complete, but are not done as for the requirements: The Team has completed nothing and all their work is wasted. If the PO had not changed the priority of the Stories half way through the Sprint, the Team would have completed and delivered Story A,

However, if a team together with the PO defines that a new prio makes more sense then a new prio should be possible.