The milestone is displayed at the top of the page (between Admin and the list of panes).
On Content and Planning v1 you can edit the milestone and switch to another "sibling" milestone (other milestones of our parent).

The main difference between sub-milestone edit and milestone edit is that, as of today, the angular application scope is only inside "Planning v2" pane (technically speaking, everthing outside the pv2 pane is manage by Tuleap regular code and, inside the pane, it's an angular application).
In order to have the same user experience for milestone and sub-milestone edit (same modal, update of page without reload) the scope of the angular application should be extended to the whole Agile Dashboard.

However, we won't rewrite every Pane in Angular (Content, Planning v1, Burndown and Cardwall), so when user will click on them, it will reload the page and display the current view for those tabs.
The warning we raise is that, until we update those Panes in Angular, they will still use the modale v1.

Concretly speaking, when this story will be done, let say you go on 8.6 milestone Content pane, you click on 8.6 to edit the date, you get the modale v1, and when you submit the page is reload. Then you click on "Planning v2", click again on 8.6 (maybe you got the date wrong again), then you will have another modal (the new one) and when you click on submit, everything is updated without page reload.