1. The PO says go.
2. The Teams says they are ready.
3. The SM has determined a time box for the sprint.
4. The team and PO agree to a time box
5. The PO understands and is prepared to talk about the stories

Comment: There are a number of things you should do before you can even begin planning.  The most important thing you can do is make sure that your Product Owner is prepared, and understands what the stories are about.  Remember that the Product Owner is a role here, so what we’re actually saying is that someone on the Team knows about each story; that is, each story has its own champion (Story Owner) who represents the Stakeholder’s needs/wants to the Team. This may require that the Product Owner (person) coordinates the efforts of all the Story Owners.

Comment: Changes to work forms an interesting tension.  At a fine grained detailed level it changes all the time. Each person’s individual to-dos often change toreflect their understanding of what it takes to get the job done. As the level of granularity increases to task then it is a change to the team’s plan. If the number of changes is significant and adds up to more than one story’s worth of work then you better stop and adjust your plan, usually you want the product owner in on that discussion. And if there are several new  stories that were  suddenly found and are so important they must be done right now, then call a stop and reset your entire sprint with a sprint planning session. Generally, the commitment by the team to the sprint should not change. Note: definition of team makes this an interesting discussion. Bottom Line: The goal here is to help the team get better at  expressing work they can do and following through on a commitment.