Editing Unwritten: System Reference Document/Dramatic Time and Milestones (section)

===Major Milestone===

A major milestone should only occur when something happens in the campaign that shakes it up a lot—the end of a story arc (or around three chapters) or any other large-scale change that reverberates around your game world.

On a major milestone, each player performs all of the actions that happen in a significant milestone. In addition, each player can:
:...rename their character’s High Concept, if they want.
:...increase their refresh by one.
:...get the benefits of a journey roll as if they succeeded at the roll (this is in addition to the roll they get from the significant milestone).

A major milestone is a pretty big deal. It should signal that lots of things in the world of your game have changed. Some of that will probably be reflected in world itself, but given the number of chances the PCs have had to revise their aspects in response to the story, you could be looking at a group with a much different set of priorities and concerns than they had when they started.