Once you have the phases/milestones set, if you only use the slider to move the master release around, all of the phases and milestones will move around with it automatically. However, if you change the date manually (clicking on the internal date, choosing a new date), they will not move around.
It be more helpful if the behavior was the same no matter how you changed the internal date.
When changing the release date it is not possible to know the users intent - whether they want release phases moved or not. There are too many cases where moving the release phases would be wrong. So we will not implement this. The workaround is to use the Gantt Chart view to move the entire release.