Sprint Backlog
<Only the Development Team can change its Sprint Backlog during a Sprint>
In case if PO wants to change the PBI in the sprint backlog, can PO change it or is this a freeze period.
If the PO wants to change what have been forecast at the sprint planning, a conversation between the PO and the Dev Team must occur. Will the changes endanger the sprint goal? Not started PBI in the sprint could be interchanged with more important requirements as a result of the conversation.
If the PO believes that the plan for achieving the Sprint Goal would be improved by changing Product Backlog items, then he or she ought to advise the Development Team accordingly. They can then make an informed decision.
Bear in mind that a Product Backlog item ought to leave room for discussion during the Sprint. For example a User Story, as a placeholder for a conversation about a possible requirement, should be compliant with the INVEST criteria and thus negotiable to some degree.