For the moment, a lot of buildings don't change of aspect when their level increase.
it's a pleasure for the player to see really the evolving of the base and not only changing in numbers (but, of course, it's a simple and good system).
On another point of view, when a vulture come to the base,
it's a good thing to not show the real level of the defenses.
So, we can imagine a camo (or lure) for building of a type to have always the same aspect (like now) whatever his level is.