Hitting the Demote button on the Milestone Page after selecting a Milestone demotes it down to the next lower level in the WBS hierarchy (so, for example, demoting a doubly-promoted Milestone from the BCP level will result in it disappearing from that level but still appearing (now singly-promoted) in both the Project and Programme levels; demoting it again, from either the Project or Programme level, would result in it appearing (as unpromoted) at the Project level only.
Note that when a Milestone is promoted it is flagged as promoted wherever it appears in the hierarchy (i.e. at the level from which it has been promoted as well as the higher levels). Thus if you see a Milestone flagged as 'promoted' you know that either it has been promoted from this level or to this level; you can tell which by examining the Promotion Pyramid.