Regular fixed length sprints (ie, 2 weeks)
Continuous flow
At the end of each sprint if approved by the product owner
Continuous delivery or at the team's discretion
Product owner, scrum master, development team
No existing roles. Some teams enlist the help of an agile coach.
Teams should strive to not make changes to the sprint forecast during the sprint. Doing so compromises learnings around estimation.
Change can happen at any time