They don't have much time to really do it to effect the least amount of people. Between spring break, summer, HHN, thanksgiving, christmas and of course the HP celebration, they don't have many options where they have a full month because those are just the major holidays. Other smaller holidays draw the crowds too.
I sort of wish, and kinda think, they designed the ride in a way to be able to do as much TLC to the ride as possible without having to shut it down. I mean, we have already lost the dragon multiple times. They can do work on the vehicles themselves without having to shut the whole ride down because they have so many vehicles and some extra. They can do work on various video screens while the ride is running by simply not having the vehicle enter a dome that is getting projector work done. They even have done stuff with the dementors without closing the ride. I am 100% sure Universal knows that this ride can't be down for long. I feel like if they are going to add anything, they will have it fully complete and ready to quickly install without having to shut it down for long. As long as they don't do anything to the ride system itself, they can take away and add things (like more dementors or rock work) without having to do a lot of testing to reopen it. They could even reduce lighting and put up black walls in the areas they are working and normal guests that have never ridden the ride would be none the wiser as to what is missing.