Do spells with Target: Boundary end if the boundary no longer applies? For example a castle is defined as a boundary by its walls. What if one of the walls falls?
Also: Is the Aegis strictly bound to something that constitutes a boundary or can the affected area be chosen relatively freely? Guardians of the Forests states that Durenmar once extended the borders of its Aegis to include a certain vis source that popped up just outside.
By building a low wall, maybe? It could go either way.
If I cast a spell with Target: Circle, it "ends if the circle is broken, even if that is before the duration of the spell expires". Such text is not found in the descriptions of Target: Room, Target: Structure or Target: Boundary. There is also no provision for a spell with Target: Group to expire once the targeted group scatters.
Given that "the target describes what the spell can affect", I do not see how a modification of the conditions defining the spell's Target after its casting should have any effect on its duration, unless this is explicitly stated (like it is for Target: Circle).
A spell with Target: Boundary requires "a well defined natural or man-made boundary" to be cast. The description of 'Aegis of the Hearth' suggests, that the Aegis ritual "often ends with the participants walking around the perimeter of the protected area in order to define it". So a wall - low or high - to define the boundary is sufficient, but not necessary: more ephemeral means to define it are quite fine.